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Teacher’s Guide

© 2014 Scholastic Education International (S) Pte LtdChapter 9: Addition and Subtraction

Chapter 9: Addition and Subtraction

Scheme of Work Total Duration: 18 h Lesson Objectives Materials Resources Vocabulary

Let’s Remember(40 min)

• To write a family of four addition and subtraction facts• To add a 1-digit number and a 2-digit number

without regrouping • To add within 1000 with regrouping• To use a part-whole bar model to represent an addition

situation • To subtract a 1-digit number from a 2-digit number

without regrouping • To subtract tens from a 2-digit number• To use a part-whole bar model to represent

a subtraction situation

• CB: pp. 7–8

Lesson 1: Finding the Missing Number 5 h 20 min

Finding the missing part in an addition sentence

• • Connecting cubes (blue, red and green)

• CB: pp. 9–10

Finding the missing part in a subtraction sentence

• • 2 boxes of crayons (10 crayons in each box)

• CB: pp. 11–12

Finding the missing whole in a subtraction sentence

• • 1 box• 24 marbles

• CB: pp. 12–13• PB: pp. 7–8

Making 100 by counting on • To make 100 by counting on • 1 copy of Hundreds Chart (TR9.1) per pair

• CB: p. 14

Making 100 using place value

• To make 100 using place value • Base ten blocks • CB: pp. 15–16• PB: pp. 9–10


Lesson Objectives Materials Resources Vocabulary

Lesson 2: Mental Addition 6 h

Adding a 1-digit number to a 2-digit number with regrouping

• To mentally add a 1-digit number to a 2-digit number with regrouping

• CB: p. 17• PB: p. 11

Adding two 2-digit numbers without regrouping

• To mentally add two 2-digit numbers without regrouping • Magnetic counters • CB: p. 18• PB: p. 12

Adding ones, tens or hundreds to a 3-digit number without regrouping

• To mentally add ones, tens or hundreds to a 3-digit number without regrouping

• Magnetic counters • Place value cards• Reusable adhesive

• CB: pp. 19–20• PB: p. 13

Adding ones to a 3-digit number with regrouping

• To mentally add ones to a 3-digit number with regrouping • CB: p. 21

Adding tens to a 3-digit number with regrouping

• To mentally add tens to a 3-digit number with regrouping • CB: p. 21• PB: p. 14

Adding 98 or 99 to a 1-digit number

• To mentally add 98 or 99 to a 1-digit number • CB: p. 22


• To mentally add 98 or 99 to a 2-digit number • CB: p. 22• PB: p. 15


• To mentally add 98 or 99 to a 3-digit number • CB: pp. 23–24• PB: p. 16

Lesson 3: Mental Subtraction 6 h

Subtracting a 1-digit number from a 2-digit number with regrouping

• To mentally subtract a 1-digit number from a 2-digit number with regrouping

• CB: p. 25• PB: p. 17

Subtracting a 2-digit number from another 2-digit number without regrouping

• To mentally subtract a 2-digit number from another 2-digit number without regrouping

• CB: p. 26• PB: p. 18


Lesson Objectives Materials Resources Vocabulary

Subtracting ones, tens or hundreds from a 3-digit number without regrouping

• To mentally subtract ones, tens or hundreds from a 3-digit number without regrouping

• Place value cards• Reusable adhesive

• CB: pp. 27–28• PB: p. 19

Subtracting ones from a 3-digit number with regrouping

• To mentally subtract ones from a 3-digit number with regrouping

• CB: p. 29

Subtracting tens from a 3-digit number with regrouping

• To mentally subtract tens from a 3-digit number with regrouping

• CB: p. 29• PB: pp. 20–21

Subtracting 98 or 99 from hundreds

• To mentally subtract 98 or 99 from hundreds • CB: p. 30

Subtracting 98 or 99 from a 3-digit number

• To mentally subtract 98 or 99 from a 3-digit number • 1 copy of Mental Addition Strategies Mind Map (TR9.2)

• 1 copy of Mental Subtraction Strategies Mind Map (TR9.3)

• CB: pp. 30–31• PB: p. 22

© 2014 Scholastic Education International (S) Pte LtdChapter 9: Addition and SubtractionC4

Chapter 9 Addition and Subtraction

Chapter OverviewLet’s RememberLesson 1: Finding the Missing Number Lesson 2: Mental Addition Lesson 3: Mental Subtraction

Note for Teachers

in addition and subtraction sentences. To do this, they

between addition and subtraction. Students also learn how to make 100 with two numbers by counting on or using place value.

Various mental addition and subtraction strategies are introduced here. Students learn how to regroup numbers to help them calculate the answers to addition and subtraction questions. These build upon the concepts of addition or subtraction with and without regrouping. Students may draw number bonds to help them in the initial stages, but eventually, they are expected to be able to do the addition and subtraction independently.

8

6. Subtract 30 from 54.

54 – 30 =

84 – 52 =

5. Subtract 3 from 45.

45 – 3 = 45 – 3 40 5

5 – 3 = 22 + 40 =

5 tens – 3 tens = tens

25 + 43 =

This is a part-whole bar model.

the whole.

4.

25 43

?

This is a part-whole bar model.We subtract one part from the

52

84

?

7.

+24

53

–85

42

68

32

68

224

32

42

42

7

Addition and SubtractionAAdAdAdAd

52 + 3 50 2

2 + 3 = 55 + 50 = 55

1. 8 + 7 = 15 7 + = 15 15 – 8 = 15 – 7 =

2. Add 52 and 3.

52 + 3 =

3. Add 58 and 80.

58 + 80 = 58 + 80 38 20

20 + 80 = 100100 + 38 =

These number sentences form a fact family.act family.a8

7

55

8

138

138

Recall:

1. Writing a family of four addition and subtraction facts (CB 1A Chapter 4)

2. Adding a 1-digit number and a 2-digit number without regrouping (CB 1B Chapter 19)

3. Adding within 1000 with regrouping (CB 2A Chapter 3)

4. Using a part-whole bar model to represent an addition situation (CB 2A Chapter 2)

5. Subtracting a 1-digit number from a 2-digit number without regrouping (CB 1B Chapter 19)

6. Subtracting tens from a 2-digit number (CB 1B Chapter 19)

7. Using a part-whole bar model to represent a subtraction situation (CB 2A Chapter 2)

© 2014 Scholastic Education International (S) Pte Ltd 5

9


You will learn to…

Lesson 1 Finding the Missing Number

part

whole

part

?

7

12

?

7 + = 12

12 – 7 =

So, 7 + = 12.

Let's draw a bar model

7 and 5

5

5

5

Lesson 1: Finding the Missing Number

Duration: 5 h 20 min


Objective:•

Materials:• Connecting cubes (blue, red and green)

Resource:• CB: pp. 9–10

Have students get into groups of four. Distribute some connecting cubes (blue, red and green) to each group. Have each group set aside 12 green and 7 blue connecting cubes. Ask them to lay the cubes on their tables as follows:

Ask: How many green cubes are there? (12) How many blue cubes are there? (7) How many red cubes do we need to join to the blue cubes to make 12 cubes? (5) Have each group check by connecting 5 red cubes to the blue cubes.Say: So, 7 and 5 make 12. Guide students through the example on CB p. 9. Relate the activity with the connecting cubes to the picture in the coursebook.Say: e ha e 7 sh e add 5 more sh to get 12 sh.


Draw the part-whole bar model as shown on CB p. 9. Write: 7 + = 12Elicit the answer from students. (5)Say: o nd the missing art, we can also subtract 7 rom 12 Write: 12 – 7 = Elicit the answer from students. (5) Have students check by recalling related addition and subtraction facts. Ask: Are 7 + 5 = 12 and 12 – 7 = 5 from the same fact family? (Yes)

the missing part in an addition sentence.


11


?

20 – = 12

12

20

?


20 – 12 =

So, 20 – = 12.

whole

?

part part

8

88

8

10

1. Complete the number sentences.

a)

b) 4 + = 13 c) 9 + = 39

d) 16 + = 52 e) 24 + = 89

f) + 14 = 60

h) + 17 = 65 i) + 52 = 90

part whole

we subtract.30 – 14 =

14 + = 30

part

30

14 ?

9 30

7

48

46

38

6536

16

16

a number sentence using subtraction.Task 1(a) provides pictorial guidance in the form of a part-whole bar model and demonstrates to students

the other missing part.

independently. Help students by drawing out the bar models if necessary.


Objective:•

Materials:• 2 boxes of crayons (10 crayons in each box)

Resource:• CB: pp. 11–12

Have students look at the example on CB p. 11. Project two boxes of crayons on the board.Ask: How many crayons are there in the two boxes? (20) Say: Now, I take some crayons away and am left with 12 crayons.Remove 8 crayons from one box of crayons. Say: et us see how we can nd the number of crayons I ha e taken awayHave a student count the number of crayons that are left. (12)drawing a part-whole bar model. Draw the part-whole bar model as shown on the page. Say: o nd the number of crayons that ha e been taken away, we subtract 12 from 2 his gi es us .

Write: 20 – 12 = Elicit the answer from students. (8)Say: o nd one art in a subtraction sentence, we subtract. Write: 20 – = 12 Guide students to derive the answer by stating that 20 – 8 = 12 and 20 – 12 = 8 are from the same fact family. Elicit the answer from students. (8) Write: 8 crayons are taken away.


13

18 6

?

18 + 6 =

So, – 6 = 18.

There were



a)

b) – 7 = 10

c) – 16 = 40

– 9 = 5

P B Chapter 9: Exercise 1

we add.5 + 9 =

part part whole

5 9

?

24

24

24

1414

17

56

12

21 – = 5

b) 20 – = 13 c) 47 – = 19

part part whole


whole part

21 – 5 =

?

– 6 = 18

21

5?


a)

part

16

7 28

24

16

subtraction sentence.

For struggling students, get them to use number bonds



Objective:•

Materials:• 1 box• 24 marbles

Resources:• CB: pp. 12–13• PB: pp. 7–8

Guide students through the example on CB p. 12. Highlight to students that we can use marbles to stand for apples. Project a box of 18 marbles and 6 loose marbles on the board.


Say: There are some a les in the box After a les are taken out from the box, 1 a les are left e can nd the number of a les in the box at rst by utting the a les back into the box

(24)

Say: e can also nd the missing whole by drawing a art whole bar model.

Have students look at the bar model on CB p. 13. Highlight to students that the missing whole is the total number of apples in the box.Ask: How can we nd the total number of a les? (Add the number of apples left in the box and the number of apples taken out)

Write: 18 + 6 = 24Say: To nd the missing whole, we add the two arts 2 – = 1 belongs to the same fact family as 1 + = 2


Go to Practice Book Chapter 9 Exercise 1 (TG p. 24).


14

?

68 + = 100

Making 100 by counting on

+ 2

68 70 100

+ 30

+ 32


a) 34 + = 100

b) + 82 = 100

34 40 100 + 6 + 60

82 90 100 + +

68 + 32 = 100

32

8 10

66

18

Making 100 by counting on

Objective:• To make 100 by counting on

Materials:• 1 copy of Hundreds Chart (TR9.1) per pair

Resource:• CB: p. 14

Guide students through the example on CB p. 14.Have students get into pairs. Distribute a copy of Hundreds Chart (TR9.1) to each pair.Say: There are beads in a bottle e need to nd how many more beads are needed to make 1

e can nd the answer by counting on.Have each pair look at the number 68 on the chart. Say: First, we count on to 70. Have students write ‘+1’ on the chart as they count on in ones from 68 to 70.Say: Then we count on by tens to 100.Have each pair write ‘+10’ on the chart as they count on in tens from 70 to 100. An extract of their hundreds chart should look like this:

68 69 70

78 79 80

88 89 90

98 99 100

+10

+10

+10

+1 +1

Ask: How many did we count on from to get to 70? (2) How many did we count on from 70 to get to 100? (30)

Write: 68 70 100+ 30

+ 32

+ 2

Say: e counted on 2 from to get to 100 So, we need to add 2 beads to beads to get 100 beads

Write: 68 + 32 = 100Say: 32 more beads are needed to make 100.

Task 1 provides practice on making 100 by counting on.

in tens to get 100.


15


?

53 + = 100100 – 53 =

5 tens 3 ones+ tens ones

9 tens 10 ones

a) 76 + = 100

b) + 9 = 100

7 tens 6 ones+ tens ones

9 tens 10 ones

9 ones+ tens one

9 tens 10 ones

4747 4 7

2

24

91

4

19

16


a) + 25 = 40 b) 23 – = 6

c) + 17 = 56 d) 43 – = 21

e) 58 + = 72 f) – 79 = 11

= 100 h) – 18 = 54

i) + 25 = 100 j) 100 – = 93

= 100 l) 100 – = 57

2. Subtract.

a) 100 – 26 = b) 100 – 61 =

c) 100 – 2 = d) 100 – 8 =

Practice 1


2. Subtract.

a) 100 – 38 b) 100 – 99

c) 100 – 98 d) 100 – 4

e) 100 – 9 f) 100 – 3

74

98

39

92

15 17

14

75 7

37

62 1

2 96

91 97

43

90

39 22

54 72


Objective:• To make 100 using place value

Materials:• Base ten blocks

Resources:• CB: pp. 15–16• PB: pp. 9–10

Use base ten blocks to represent the number 53, and project them on the board.Say: e can also use lace alue to hel us make 100 There are 5 tens and 3 ones here et us look at the ones rst e can add 7 ones to 3 ones to make 10 ones

Connect 7 unit cubes to the 3 unit cubes. Say: e now ha e 5 tens and 10 ones Ask: What number do the base ten blocks show? (60) How many more tens do we need to add to make 100? (4)Add 4 ten-rods to the existing ten-rods to make 100 cubes in total. Get students to see that 9 tens and 10 ones make 100.

Write: 53 + = 100 Write the addition in vertical form as shown on CB p. 15.


Ask: How many ones did we add to 3 ones to make 10 ones? (7) How many tens did we add to 5 tens to make 9 tens? (4)Complete the addition in vertical form and the addition sentence.Write: 100 – 53 = Elicit the answer from students. (47)Say: 7 and 53 make 100 So, 7 must be added to 53 to make 100

Task 1 provides practice on completing an addition sentence by making 100.

Task 2 provides practice on solving a subtraction sentence by making 100.


Practice 1

an addition or subtraction sentence. Tasks 1(g) and 1(i)–1(l) require students to complete the number sentences by making 100.

Task 2 provides practice on solving a subtraction sentence by making 100.


17



Lesson 2 Mental Addition

Add 36 and 9.

36 + 9 = 36 + 9 4 536 + 4 = 4040 + 5 =

1. Add.

a) 49 + 4 = b) 38 + 7 =

c) 56 + 5 = d) 87 + 6 =

e) 74 + 8 = f) 63 + 9 =


45

45

53 45

61 93

82 72

Lesson 2: Mental Addition

Duration: 6 h


Objective:• To mentally add a 1-digit number to a 2-digit number

with regrouping

Resources:• CB: p. 17• PB: p. 11

Have students look at the example on CB p. 17. Write: 36 + 9 Underline the digits in the ones place for both numbers. Ask: o ones and 9 ones add u to more than 9 ones? (Yes) Say: We can rst make a ten with the ones in the 2 digit number to hel us nd the answer. Ask: What can be added to 3 to make 0? (4)

Write: 36 + 9 4 ?

36 + 4 = 40

Ask: What number makes 9 with 4? (5)Say: We add 5 to 40Write: 40 + 5 = Elicit the answer from students. (45) Say: So, 3 + 9 = 45

Task 1 provides practice on mentally adding a 1-digit number to a 2-digit number with regrouping. Students should make a ten with the ones in the 2-digit number, before adding the remaining part of the 1-digit number to the result.



18

Add 43 and 26.

43 63 69

43 + 26 =

43 + 26 20 643 + 20 = 6363 + 6 = 69

1. Add.

a) 65 + 12 = b) 51 + 18 =

c) 44 + 54 = d) 76 + 23 =

e) 32 + 47 = f) 24 + 35 =


+ 20 + 6

43

26


69

77 69

98 99

79 59


Objective:• To mentally add two 2-digit numbers

without regrouping

Materials:• Magnetic counters


Refer students to the example on CB p. 18. Draw a place value chart on the board as shown below:

2-digit numbers

Tens Ones

43

26

Have a student place magnetic counters in each column. Prompt the student if necessary, asking him/her how many tens and ones there are in each number. The completed place value chart should look like this:

2-digit numbers

Tens Ones

43

26

Ask: How many tens do we have in total? (6) How many ones do we have in total? (9) What number do

tens 9 ones make? (69)Say: We know that 2 has 2 tens and ones So, adding 43 and 2 is the same as adding 2 tens to 43, then adding ones to the result.

Write: 43 + 26 20 6

Ask: What is 43 + 20? (63) What is 3 + ? (69)

Write:

43 63 69+ 6

+ 26

+ 20

Say: So, 43 + 2 = 9


Task 1 provides practice on mentally adding two 2-digit numbers without regrouping. Students are expected to split one of the numbers into tens and ones, then add the tens and ones to the other number separately.



19


a) Add 264 and 3.

b) Add 430 and 50.

2 hundreds 6 tens 4 ones

2 0 0

46 0

2 0 0

?6 0

+ 3

4 ones + 3 ones = ones

3 tens + 5 tens = tens

s


4 0 0 4 0 0?3 0 + 50

430 + 50 =

264 + 3 =

480

267

8

7


Objective:• To mentally add ones, tens or hundreds to a

3-digit number without regrouping

Materials:• Magnetic counters• Place value cards• Reusable adhesive

Resources:• CB: pp. 19–20 • PB: p. 13

(a)

Say: et us add 2 4 and 3 mentally. Draw an empty place value chart on the board as shown below:

Hundreds Tens Ones

Have a student place magnetic counters in the hundreds, tens and ones columns to show 264. The completed place value chart should look like this:

Hundreds Tens Ones

Stick the place value cards ‘200’, ‘60’ and ‘4’ on the board as shown on the page. Add 3 magnetic counters in the ones column of the place value chart to demonstrate the addition as shown below:

Hundreds Tens Ones

Ask: What do 4 ones and 3 ones make? (7 ones) Replace the place value card ‘4’ with the place value card ‘7’. Highlight to students that this addition does not affect the number of hundreds and tens.

Write: 264 + 3 = 267 Underline the digits in the ones place. Help students see that when they add ones to a 3-digit number without regrouping, only the number of ones in the 3-digit number changes.

(b)Have students look at the second example on the page. Highlight to them that we are adding tens to a 3-digit number.

Take down the existing magnetic counters in the place value chart. Have a student place magnetic counters to show 430. The completed place value chart should look like this:

Hundreds Tens Ones

Stick the place value cards ‘400’ and ‘30’ on the board as shown on the page. Have a student add magnetic counters to show the addition of 50 on the place value chart as shown below:

Hundreds Tens Ones

Ask: What do 3 tens and 5 tens make? (8 tens)Replace the place value card ‘30’ with the place value card ‘80’. Highlight to students that the number of hundreds and ones do not change.Write: 430 + 50 = 480 Underline the digits in the tens place. Help students see that when they add tens to a 3-digit number without regrouping, only the number of tens in the 3-digit number changes.


20

c) Add 342 and 300.

3 hundreds + 3 hundreds = hundreds

1. Add.

a) 153 + 4 = b) 282 + 7 =

c) 320 + 60 = d) 436 + 50 =

e) 200 + 200 = f) 500 + 300 =

h) 504 + 300 =

3 0 0

24 0

?4 0

2

+ 300


342 + 300 =


6

642

157 289

400 800

380 486

456 804

(c)

Have students look at the example on CB p. 20. Highlight to them that we are adding hundreds to a 3-digit number. Take down the existing magnetic counters in the place value chart. Have a student place magnetic counters to show 342. The completed place value chart should look like this:

Hundreds Tens Ones

Stick the place value cards ‘300’, ‘40’ and ‘2’ on the board as shown on the page. Add 3 magnetic counters in the hundreds column to show the addition as shown below:

Hundreds Tens Ones

Ask: How many hundreds are there now? (6)Replace the place value card ‘300’ with the place value card ‘600’. Get students to notice that the number of tens and ones do not change.

Write: 342 + 300 = 642 Underline the digits in the hundreds place. Help students see that when they add hundreds to a 3-digit number without regrouping, only the number of hundreds in the 3-digit number changes.


Task 1 provides practice on mentally adding ones, tens or hundreds to a 3-digit number without regrouping. In Tasks 1(a) and 1(b), students are expected to add ones to a 3-digit number. In Tasks 1(c) and 1(d), students are expected to add tens to a 3-digit number. In Tasks 1(e)–1(h), students are expected to add hundreds to a 3-digit number.

Provide place value charts and counters for struggling students, if necessary.



21


Add 425 and 8.

425 + 8 = 425 + 8 5 3425 + 5 = 430430 + 3 =

1. Add.

a) 278 + 5 = b) 607 + 4 =

c) 356 + 9 = d) 517 + 6 =

1. Add.

a) 235 + 90 = b) 164 + 60 =

c) 476 + 80 = d) 358 + 70 =


Add 261 and 60.

261 + 60 = 261 + 60 221 4040 + 60 = 100100 + 221 =


433

433

283 611

365 523

325 224

556 428

321

321


Objective:• To mentally add ones to a 3-digit number

with regrouping


Refer students to CB p. 17. Help students recall adding a 1-digit number to a 2-digit number with regrouping. Highlight to them that adding a 1-digit number to a

a ten with the ones in the 3-digit number. Refer them back to CB p. 21. Write: 425 + 8 Ask: What do we add to 425 to make 430? (5)

Write: 425 + 8

5 ?

425 + 5 = 430

Ask: What number makes with 5? (3) Say: We add 3 to 430. Write: 430 + 3 = Elicit the answer from students. (433)Say: So, 425 + = 433

Task 1 provides practice on mentally adding ones to a 3-digit number with regrouping. Students should make a ten with the ones in the 3-digit number, before adding the remaining ones of the 1-digit number to the result.

Guide struggling students by drawing out the number bonds for the ones in each question.


Objective:• To mentally add tens to a 3-digit number

with regrouping


Refer students to the next example on CB p. 21.Write: 261 + 60 Have students look at the number of tens for both numbers.

Ask: oes adding tens and tens make more than 9 tens? (Yes)

Ask: What makes 100 with 0? (40)

Write: 261 + 60 ? 40

60 + 40 = 100

Ask: What number makes 2 1 with 40? (221) What is 100 + 221? (321)Say: So, 2 1 + 0 = 321

Task 1 provides practice on mentally adding tens to a 3-digit number with regrouping. Students should make 100 with the tens, before adding the remaining part of the 3-digit number to 100.

Guide struggling students by drawing out the number bonds for the 3-digit number in each question.



22

1. Add.

a) 99 + 2 = b) 99 + 9 =

c) 98 + 5 = d) 7 + 98 =

1. Add.

a) 99 + 46 = b) 98 + 23 =

c) 57 + 98 = d) 68 + 99 =



Add 99 and 4.

99 + 4 =

Add 98 and 36.

98 + 36 =

99 + 4 1 399 + 1 = 100100 + 3 =

98 + 36 2 3498 + 2 = 100100 + 34 =


101 108

103

103

103

145

155

121

167

134

134

105


Objective:• To mentally add 98 or 99 to a 1-digit number


Refer students to the example on CB p. 22.Write: 99 + 4 Highlight to students that this addition method is similar to what they have learned previously on adding tens to a 3-digit number with regrouping. Ask: What do we add to 99 to make 100? (1)

Write: 99 + 4 1 ?

99 + 1 = 100

Ask: What is the number that makes 4 with 1? (3) What do we get when we add 3 to 100? (103)Conclude that 99 + 4 = 103.

Provide base ten blocks for struggling students to aid them in understanding the method taught.

Task 1 provides practice on mentally adding 98 or 99 to a 1-digit number. For Tasks 1(a) and 1(b), students are expected to mentally add 99 to a 1-digit number. They can do this by adding 1 to 99 to make 100, before adding the remaining part of the 1-digit number to 100.For Tasks 1(c) and 1(d), students are expected to mentally add 98 to a 1-digit number. They can do this by adding 2 to 98 to make 100, before adding the remaining part of the 1-digit number to 100.




Refer students to the next example on CB p. 22. Write: 98 + 36 Explain to students that this addition method is similar

to make 100 with 98 or 99, before adding the remaining part of the 2-digit number to 100. Ask: What do we add to 9 to make 100? (2)


Write: 98 + 36 2 ?

98 + 2 = 100

Have a student write the missing number. (34) Ask: What do we do next? (Add 34 to 100) What is 100 + 34? (134) Conclude that 98 + 36 = 134.

Task 1 provides practice on mentally adding 98 or 99 to a 2-digit number.For Tasks 1(a) and 1(d), students are expected to mentally add 99 to a 2-digit number. They can do this by adding 1 to 99 to make 100, before adding the remaining part of the 2-digit number to 100. For Tasks 1(b) and 1(c), students are expected to mentally add 98 to a 2-digit number. They can do this by adding 2 to 98 to make 100, before adding the remaining part of the 2-digit number to 100.



24

3. Add mentally.

a) 781 + 5 b) 292 + 7

c) 364 + 3 d) 316 + 70

e) 502 + 40 f) 625 + 30

i) 400 + 500 j) 705 + 200

4. Add mentally.

a) 234 + 8 b) 299 + 5

c) 658 + 7 d) 365 + 80

e) 287 + 40 f) 480 + 50

5. Add mentally.

a) 99 + 5 b) 62 + 99

c) 86 + 98 d) 99 + 98

e) 98 + 304 f) 99 + 652

786 299

367

542

242

161104

184

304

700

665

197

402

445

700

900

327

404

751

397330

530

805

905

852 898

386

655

23


Practice 2

Add 237 and 99.

237 + 99 = 237 + 99 236 11 + 99 = 100100 + 236 =

1. Add.

a) 256 + 99 = b) 463 + 98 =

c) 342 + 98 = d) 536 + 99 =

1. Add mentally.

a) 46 + 9 b) 58 + 4

c) 29 + 5 d) 37 + 8

e) 65 + 6 f) 84 + 7

2. Add mentally.

a) 73 + 16 b) 35 + 21

c) 24 + 45 d) 52 + 25

e) 67 + 32 f) 13 + 84


336

336

355 561

440

55

89

62

56

34

69

45

77

71

99

91

97

635



Resources:• CB: pp. 23–24• PB: p. 16

Have students look at the example on CB p. 23. Write: 237 + 99

answer. Prompt them to recall what they learned earlier. Lead them to see that they should also make 100 with 99, then add the remaining part of 237 to 100. Have a student draw the number bonds for 237. Prompt him/her, if necessary, by asking him/her what makes 100 with 99 (1) and what makes 237 with 1. (236) The correct number bond is shown below:

237 + 99 236 1

Write: 1 + 99 = 100 100 + 236 = Elicit the answer from students. (336) Conclude that 237 + 99 = 336.

Task 1 provides practice on mentally adding 98 or 99 to a 3-digit number. For Tasks 1(a) and 1(d), students are expected to mentally add 99 to a 3-digit number. They can do this by adding 1 to 99 to make 100, before adding the remaining part of the 3-digit number to 100.For Tasks 1(b) and 1(c), students are expected to mentally add 98 to a 3-digit number. They can do this by adding 2 to 98 to make 100, before adding the remaining part of the 3-digit number to 100.


Practice 2Task 1 provides practice on mentally adding a 1-digit number to a 2-digit number with regrouping.

Task 2 provides practice on mentally adding two 2-digit numbers without regrouping.

Task 3 provides practice on mentally adding ones, tens or hundreds to a 3-digit number without regrouping.

Task 4 provides practice on mentally adding ones or tens to a 3-digit number with regrouping.

Task 5 provides practice on mentally adding 98 or 99 to a 1-digit, 2-digit or 3-digit number.


25



Lesson 3 Mental Subtraction

a) Subtract 4 from 32.

32 – 4 =

b) Subtract 6 from 70.

70 – 6 =

32 – 4 22 1010 – 4 = 66 + 22 =

70 – 6 60 1010 – 6 = 44 + 60 =

1. Subtract.

a) 24 – 6 = b) 51 – 3 =

c) 70 – 9 = d) 60 – 8 =


28

64

64

28

18 48

61 52

Lesson 3: Mental Subtraction


Duration: 6 h


Objective:• To mentally subtract a 1-digit number from

a 2-digit number with regrouping

Resources:• CB: p. 25 • PB: p. 17

(a)

Write: 32 – 4 Have students look at the ones for both numbers. Ask: Can we subtract 4 ones from 2 ones? (No) Highlight to students that since we cannot subtract directly, we split 32 into 10 and another number as follows:

32 – 4 22 10

Say: We rst subtract 4 from 10Ask: What is 10 – 4? (6) Write: 10 – 4 = 6Say: Now, we add 22 to to nd the answer.Write: 6 + 22 = Elicit the answer from students. (28)Say: Therefore, 32 – 4 = 2 .

(b)Have students look at the second example on the page. Write: 70 – 6 Have a student draw the number bond for 70 in a way that allows him/her to subtract 6 ones from 10 ones. Prompt him/her, if necessary, by asking what number makes 70 with 10. (60) The number bond should look like this:

70 – 6 60 10

Say: First, we subtract from 10, which gives us 4. Ask: What do we do next? (Add 60 to 4) What does that give us? (64)Say: So, 70 – = 4.

Struggling students might be confused and subtract 4 from 60, instead of add 60 to 4. Use base ten blocks to demonstrate the logic behind this strategy for these students.

Task 1 provides practice on mentally subtracting a 1-digit number from a 2-digit number with regrouping.

the answers. They can draw number bonds to help them visualize the subtraction.



26


Subtract 23 from 54.

54 – 23 20 354 – 20 = 3434 – 3 = 31

1. Subtract.

a) 58 – 17 = b) 87 – 16 =

c) 65 – 42 = d) 75 – 55 =

e) 49 – 32 = f) 63 – 21 =

5 tens 4 ones

54 – 20 34 – 3 31

54 – 23 =


7141

31

20

42

23

17


Objective:• To mentally subtract a 2-digit number from another

2-digit number without regrouping

Resources:• CB: p. 26 • PB: p. 18

Refer students to CB p. 26. Say: We want to subtract 23 from 54Draw a place value chart on the board as shown below:

Tens Ones

Have a student draw circles in each column to show the number 54. The completed place value chart should look like this:

Tens Ones

Say: We are going to use the lace value chart to hel us subtract 23 is 2 tens 3 ones We can subtract the tens and ones se arately et us subtract the tens rst. Strike out 2 tens from the place value chart as shown below:

Tens Ones

Say: We are left with 3 tens 4 ones Now, let us look at the ones We need to subtract 3 ones from 4 ones. Have a student strike out 3 ones in the place value chart as shown below:

Tens Ones

Ask: How many tens are left? (3) How many ones are left? (1) What number are we left with? (31)Say: We know that 23 has 2 tens 3 ones So, subtracting 23 from 54 is the same as subtracting 2 tens from 54, then subtracting 3 ones from the result

Write: 54 – 23 20 3

Ask: What is 54 – 20? (34) What is 34 – 3? (31)

Write:

54 34 31– 3

– 23

– 20

Say: So, 54 – 23 = 31

Task 1 provides practice on mentally subtracting a 2-digit number from another 2-digit number without regrouping. Students can derive the answer with the help of number bonds.



27


a) Subtract 4 from 235.

b) Subtract 50 from 480.

5 ones – 4 ones = one

8 tens – 5 tens= tens

4 0 08 0 ?– 50

4 0 0

2 0 0

53 0

?3 0

– 4

2 0 0

235 – 4 =



480 – 50 =

231

4303

1



Objective:• To mentally subtract ones, tens or hundreds from

a 3-digit number without regrouping


Materials:• Place value cards • Reusable adhesive

(a)

Say: We want to subtract 4 from 235 Highlight to students that they can use the strategy that they used to mentally subtract a 2-digit number from another 2-digit number. Draw a place value chart, with the hundreds, tens and ones columns, on the board. Have a student draw circles in each column to show the number 235. The completed place value chart should look like this:

Hundreds Tens Ones

Stick the place value cards ‘200’, ‘30’ and ‘5’ on the board as shown in (a) on CB p. 27.Say: We want to subtract 4 ones So, we look at the ones columnStrike out 4 ones from the place value chart as shown below:

Hundreds Tens Ones

Ask: What do we get when we subtract 4 ones from 5 ones? (1 one) Replace the place value card ‘5’ with the place value card ‘1’.Ask: How many hundreds are left? (2) How many tens are left? (3) How many ones are left? (1) What number are we left with? (231)

Write: 235 – 4 = 231 Underline the digit in the ones place in each number as shown above. Highlight to students that when they subtract ones from a 3-digit number without regrouping, only the number of ones in the 3-digit number changes.

(b)Erase the circles from the place value chart.Say: Now, we want to subtract 50 from 4 0 mentally.

Have a student draw circles in the place value chart to show the number 480. The completed place value chart should look like this:

Hundreds Tens Ones

Stick the place value cards ‘400’ and ‘80’ on the board as shown in (b). Strike out 5 tens from the place value chart to demonstrate the subtraction as follows:

Hundreds Tens Ones

Ask: What number are we left with after subtracting 5 tens? (430) What do you notice about the number of hundreds and ones? (They remain the same)Replace the place value card ‘80’ with the place value card ‘30’.

Write: 480 – 50 = 430 Underline the digits in the tens place in each number as shown above. Reiterate that when we subtract tens from a 3-digit number without regrouping, only the number of tens in the 3-digit number changes.


28

c) Subtract 300 from 647.

6 hundreds – 3 hundreds= hundreds

1. Subtract.

a) 218 – 5 = b) 347 – 3 =

c) 780 – 60 = d) 286 – 50 =

e) 500 – 200 = f) 700 – 400 =

h) 908 – 300 =

6 0 0

74 0

?4 0

– 300

7

647 – 300 =



3

347

213 344

300 300

720 236

342 608

(c)

Erase the circles in the place value chart. Say: Now, we want to subtract hundreds from a 3 digit number. Have a student draw circles in the place value chart to show the number 647. The completed place value chart should be as follows:

Hundreds Tens Ones

Stick the place value cards ‘600’, ‘40’ and ‘7’ on the board as shown on CB p. 28.Ask: How many hundreds do we have to subtract from 47? (3)

Demonstrate the subtraction by striking out 3 circles in the hundreds place as shown below:

Hundreds Tens Ones

Ask: What do we get when we subtract 3 hundreds from 47? (347) What do you notice about the number of tens

and ones? (They remain the same)Replace the place value card ‘600’ with the place value card ‘300’.

Write: 647 – 300 = 347 Underline the digits in the hundreds place for each number as shown above. Once again, reiterate that when we subtract hundreds from a 3-digit number without regrouping, only the number of hundreds in the 3-digit number changes.

Task 1 provides practice on mentally subtracting ones, tens or hundreds from a 3-digit number without regrouping. Tasks 1(a) and 1(b) involve subtracting ones from a 3-digit number without regrouping. Tasks 1(c) and 1(d) involve subtracting tens from a 3-digit number without regrouping. Tasks 1(e)–1(h) involve subtracting hundreds from a 3-digit number without regrouping.



29



362 – 5 =

1. Subtract.

a) 230 – 7 = b) 206 – 9 =

c) 315 – 6 = d) 481 – 4 =



527 – 40 = 527 – 40 427 100100 – 40 = 6060 + 427 =

1. Subtract.

a) 214 – 30 = b) 367 – 90 =

c) 451 – 70 = d) 540 – 80 =

362 – 5 352 1010 – 5 = 55 + 352 =


357

357

184

223 197

277

381

309

487

487

477

460



Objective:• To mentally subtract ones from a 3-digit number

with regrouping


Say: Let us learn how to mentally subtract ones from a 3 digit number with regrou ingWrite: 362 – 5 Have students look at the number of ones in both numbers.Ask: Can we subtract 5 ones from 2 ones? (No)Say: So, we can s lit the 3 digit number into 10 and another number so that we can subtract 5 from 10. Have students refer to CB p. 25 to recall the strategy learned when subtracting ones from a 2-digit number with regrouping. Say: Subtracting ones from a 3 digit number is similar to what we have learned earlier We rst s lit 3 2 in a way that allows us to subtract 5 ones from 10 ones

Write: 362 – 5 ? 10

Ask: What is 10 – 5? (5) Write: 10 – 5 = 5 Ask: What makes 3 2 with 10? (352) Say: We then add 352 to 5 to nd the answerWrite: 5 + 352 = Elicit the answer from students. (357) Conclude that 362 – 5 = 357.

Task 1 provides practice on mentally subtracting ones from a 3-digit number with regrouping. Students are expected to split the 3-digit number into 10 and another number. They should then subtract the ones from 10, before adding the remaining part of the 3-digit number to the result to get the answer. Students can use number bonds to help them.


Objective:• To mentally subtract tens from a 3-digit number

with regrouping

Resources:• CB: p. 29 • PB: pp. 20–21

Say: Now, we will learn how to mentally subtract tens from a 3 digit number with regrou ing

Write: 527 – 40 Have students look at the number of tens in both numbers. Ask: Can we subtract 4 tens from 2 tens? (No) Say: We need to s lit 527 into 100 and another number so that we can subtract 40 from 100.

Write: 527 – 40 ? 100

Ask: What number makes 527 with 100? (427) What is 100 – 40? (60) Write: 100 – 40 = 60 Ask: What do we do next? (Add 427 to 60)Write: 60 + 427 = Elicit the answer from students. (487) Conclude that 527 – 40 = 487.

Task 1 provides practice on mentally subtracting tens from a 3-digit number with regrouping. Students are to

100 and another number, so that they can subtract the tens from 100 before adding the remaining part of the 3-digit number to the result.

Go to Practice Book Chapter 9 Exercise 12 (TG pp. 30–31).


30




300 – 99 =


203 – 98 =

300 – 99 200 100100 – 99 = 11 + 200 =

203 – 98 103 100100 – 98 = 22 + 103 =

1. Subtract.

a) 200 – 99 = b) 700 – 98 =

1. Subtract.

a) 102 – 99 = b) 321 – 98 = P B Chapter 9: Exercise 13

201

201

602

105

2233

105

101


Objective:• To mentally subtract 98 or 99 from hundreds


Write: 300 – 99 Have students split 300 into 100 and another number. Point out to them that it is easier to subtract 99 from 100, and get the answer by adding the remaining part of 300 to the result.

Write: 300 – 99 ? 100

Ask: What makes 300 with 100? (200)Have students subtract 99 from 100. (1)Ask: What should we do next? (Add 200 to 1)What is 1 + 200? (201)Conclude that 300 – 99 = 201.

Task 1 provides practice on mentally subtracting 98 or 99 from hundreds. Task 1(a) requires students to mentally subtract 99 from hundreds. Students are to split the hundreds into 100 and another number, so that they can subtract 99 from 100, before adding the remaining part of the hundreds to the result.Task 1(b) requires students to mentally subtract 98 from hundreds. Students are to split the hundreds into 100 and another number, so that they can subtract 98 from 100, before adding the remaining part of the hundreds to the result.


Objective:• To mentally subtract 98 or 99 from a 3-digit number

Materials:• 1 copy of Mental Addition Strategies Mind Map (TR9.2)• 1 copy of Mental Subtraction Strategies Mind Map

(TR9.3)


Write: 203 – 98 Highlight to students that this concept is similar to what they learned earlier on subtracting 98 or 99 from hundreds. Have a student split 203. Prompt him/her, if necessary, by asking him/her what number makes 203 with 100. (103)

The number bond should be as follows:

203 – 98 103 100

Have students subtract 98 from 100 to get 2. Ask them for the next step. (Add 103 to 2 to get 105) Conclude that 203 – 98 = 105.

Task 1 provides practice on mentally subtracting 98 or 99 from a 3-digit number. Task 1(a) requires students to mentally subtract 99 from a 3-digit number. Students are to split the 3-digit number into 100 and another number, so that they can subtract 99 from 100, before adding the remaining part of the 3-digit number to the result.Task 1(b) requires students to mentally subtract 98 from a 3-digit number. Students are to split the 3-digit number into 100 and another number, so that they can subtract 98 from 100, before adding the remaining part of the 3-digit number to the result.



31

Practice 31. Subtract mentally.

a) 43 – 7 b) 56 – 9 c) 62 – 5

d) 50 – 4 e) 60 – 3 f) 80 – 6

2. Subtract mentally.

a) 42 – 12 b) 36 – 25 c) 74 – 32

d) 58 – 36 e) 89 – 47 f) 67 – 21


a) 266 – 6 b) 379 – 4 c) 517 – 2

d) 751 – 20 e) 682 – 50 f) 345 – 30


a) 411 – 8 b) 105 – 7 c) 406 – 9

d) 420 – 80 e) 218 – 40 f) 345 – 60


a) 400 – 99 b) 900 – 99 c) 800 – 98

d) 308 – 99 e) 554 – 98 f) 930 – 98

36

30

260 375 515

11 42

47 57

46

22

731

600 300 400

39798403

301 801 702

212 261 109

285178340

209 456 832

632 315

42 46

57 74

Practice 3Task 1 provides practice on mentally subtracting a 1-digit number from a 2-digit number with regrouping. Students are to split the 2-digit number into 10 and another number such that they can subtract the ones from 10, before adding the remaining part of the 2-digit number to the result.

Task 2 provides practice on mentally subtracting a 2-digit number from another 2-digit number without regrouping.

Task 3 provides practice on mentally subtracting ones, tens or hundreds from a 3-digit number without regrouping.

Task 4 provides practice on mentally subtracting ones or tens from a 3-digit number with regrouping. Tasks 4(a)–4(c) require students to split the 3-digit number into 10 and another number so that they can subtract the ones from 10, before adding the remaining part of the 3-digit number to the result. Tasks 4(d)–4(f) require students to split the 3-digit number into 100 and another number so that they can subtract the tens from 100, before adding the remaining part of the 3-digit number to the result.

Task 5 provides practice on mentally subtracting 98 or 99 from hundreds or a 3-digit number.Tasks 5(a)–5(c) provide practice on mentally subtracting 98 or 99 from hundreds.Tasks 5(d)–5(f) provide practice on mentally subtracting 98 or 99 from a 3-digit number.


Reiterate the following points:

• we subtract.

• we subtract.

• we add.

• We can count on or use place value to make 100.

• We can use different strategies to do mental addition and subtraction (Refer to TR9.2 and TR9.3).


8 9 Addition and Subtraction


68 + = 88

– 30 = 50 94 – = 49

+ 35 = 80

20 + = 90

15 + = 50 58 – = 18

– 15 = 15

+ 16 = 91 – 35 = 65

a)

c)

e)

g)

i)

b)

d)

f)

h)

j)

20

35

45

75

80 45

100

30

40

70

79 Addition and Subtraction

Exercise 1 Finding the Missing Number1. Complete the number sentences.

a)

16 + = 28

b)

18 – = 6

c)

– 7 = 9

Addition and Subtraction

E

AdAdAAAdd

12

12

16

Practice Book Exercise 1

Task Objectives Skills

1or subtraction sentence

Students are to identify the whole and parts in an addition or subtraction sentence. Pictorial guidance is given; the whole and

by counting the number of items.

2or subtraction sentence number in an addition or subtraction sentence without pictorial

guidance. Students are expected to use the part-whole method

© 2014 Scholastic Education International (S) Pte Ltd 25Practice Book: Exercises


3. Subtract.

4. Subtract.

a) 100 – 98 = b) 100 – 93 =

c) 100 – 85 = d) 100 – 27 =

e) 100 – 79 = f) 100 – 56 =

g) 100 – 22 = h) 100 – 34 =

i) 100 – 9 = j) 100 – 7 =

k) 100 – 1 = l) 100 – 4 =

100 – 20

100 – 40

100 – 10

100 – 70

a)

c)

b)

d)

80

60 30

2

15

21

78

91

99

7

73

44

66

93

96

90


Exercise 2 Finding the Missing Number1. Fill in the blanks.


a) 99 + = 100 b) 95 + = 100

c) 96 + = 100 d) 91 + = 100

e) 80 + = 100 f) 35 + = 100

g) 84 + = 100 h) 63 + = 100

i) + 42 = 100 j) + 78 = 100

k) + 6 = 100 l) + 8 = 100

m) + 27 = 100 n) + 49 = 100

90 +

60 +

5 +

+ 30

+ 98

+ 15

100

10

85

40 2

1

4

20

16

58

94

73 51

5

9

65

37

22

92

95

70



1–2 To make 100 Students are expected to count on or use place value to help

part in each addition sentence.

3–4 To make 100 Students are expected to subtract the given part from 100



Exercise 4 Mental Addition1. Fill in the missing numbers.

a) 73

3

b)

90

94

c)

80 9

d)

40

2. Fill in the missing numbers.

a)

+ 32

+ 30

40

+ 2

b)

+ 54

+ 50

45

+ 4

c)

+ 23

+ 20

52

+ 3

d)

+ 35

+ 30

64

+ 5

3. Add mentally.

a) 26 + 31 = b) 14 + 52 =

c) 53 + 34 = d) 25 + 23 =

e) 77 + 12 = f) 86 + 13 =

6

70

70

72

72

57

87

89

66

48

99

75

94

99

99

95

4

4689


Exercise 3 Mental Addition1. Add mentally.

a) 53 + 8 =

b) 49 + 6 =

2. Add mentally.

a) 28 + 6 = b) 63 + 8 =

c) 19 + 9 = d) 74 + 6 =

e) 32 + 8 = f) 55 + 9 =

g) 45 + 6 = h) 88 + 9 =

i) 56 + 7 = j) 49 + 5 =

53 + 8 1

53 + =

+ 1 =

49 + 6

49 + =

+ =

61

55

34

28

40

51

63

71

7

11 50

50 5 55

5

7 6060 61

80

64

97

54



1 To mentally add a 1-digit number to a 2-digit number with regrouping bond and to make a ten with the ones in the 2-digit number.

They should then add the remaining part of the 1-digit number to the result.

2 To mentally add a 1-digit number to a 2-digit number with regrouping may draw number bonds to help them, if necessary.



1 a number bond

Students are to recognize that each 2-digit number is split into tens and ones.

2 To mentally add two 2-digit numbers without regrouping

Students are required to follow the direction of the arrows and add the given values to the previous 2-digit numbers. They should see that adding a 2-digit number without regrouping is the same as adding the tens and ones of the 2-digit number separately.

3 To mentally add two 2-digit numbers without regrouping

the answers.




a) 135 + 6 =

b) 187 + 9 = c) 354 + 8 =

d) 408 + 7 = e) 563 + 9 =

f) 656 + 8 = g) 738 + 5 =

h) 289 + 9 = i) 827 + 4 =

2. Add mentally.

a) 163 + 50 =

b) 250 + 60 = c) 410 + 90 =

d) 638 + 90 = e) 545 + 70 =

f) 386 + 80 = g) 759 + 80 =

h) 875 + 80 = i) 775 + 70 =

135 + 6 5 1

135 + 5 =

+ 1 =

163 + 50 113 50

50 + 50 =

+ 113 =

362

572

743

831

196

415

664

298

500

615

839

845

310

728

466

955

140141140

100100 213

213

141


Exercise 5 Mental Addition1. Add mentally. Example 163 + 3 =

a) 230 + 5 = b) 405 + 4 =

c) 403 + 4 = d) 782 + 6 =

e) 652 + 7 = f) 366 + 3 =

2. Add mentally. Example 240 + 20 =

a) 519 + 30 = b) 442 + 40 =

c) 608 + 50 = d) 735 + 30 =

e) 345 + 30 = f) 833 + 60 =

3. Add mentally. Example 100 + 300 =

a) 200 + 600 = b) 300 + 200 =

c) 350 + 100 = d) 409 + 500 =

e) 415 + 300 = f) 535 + 400 =

166

260

4001 hundred + 3 hundreds = hundreds

3 ones + 3 ones = ones6

4 tens + 2 tens = tens6

4

235

407

659

409

788

369

549

658

375

482

765

893

800

450

715

500

909

935



1 To mentally add ones to a 3-digit number without regrouping

Students should recognize that when they add ones to a 3-digit number without regrouping, only the number of ones in the 3-digit number changes. An example is provided to guide them.

2 To mentally add tens to a 3-digit number without regrouping

Students should recognize that when they add tens to a 3-digit number without regrouping, only the number of tens in the 3-digit number changes. An example is provided to guide them.

3 To mentally add hundreds to a 3-digit number without regrouping

Students should recognize that when they add hundreds to a 3-digit number without regrouping, only the number of hundreds in the 3-digit number changes. An example is provided to guide them.



1 To mentally add ones to a 3-digit number with regrouping

Students are to split the ones so as to make 10 with the ones of the 3-digit number. They should then add the remaining part of the ones to the result. Guidance is provided for Task 1(a).

2 To mentally add tens to a 3-digit number with regrouping

Students are to split the 3-digit number so as to make 100 with the tens. They should then add the remaining part of the 3-digit number to the result. Guidance is provided for Task 2(a).




a) 183 + 99 =

b) 246 + 98 =

c) 199 + 99 =

d) 206 + 98 =

e) 99 + 556 =

f) 98 + 235 =

g) 99 + 408 =

h) 98 + 399 =

183 + 99 182 1

1 + 99 =

+ 182 =

282

344

298

304

655

333

507

497

100100 282



2. Add mentally.

a) 99 + 37 =

b) 53 + 99 =

c) 98 + 46 =

d) 65 + 98 =

99 + 2

98 + 4

99 + 6

98 + 3

99 + 5

98 + 7

99 + 2 1 1

99 + 1 =

+ 1 =

99 + 37 1 36

99 + 1 =

+ 36 =

101

101

102

105

105

104

136

144

163

152

100100 101

100136100



1 To mentally add 98 or 99 to a 1-digit number

Students are to split the 1-digit number so as to make 100 with 98 or 99. They should then add the remaining part of the 1-digit number to the result. Guidance is provided for Task 1(a).


Students are to split the 2-digit number so as to make 100 with 98 or 99. They should then add the remaining part of the 2-digit number to the result. Guidance is provided for Task 2(a).




Students are to split the 3-digit number so as to make 100 with 98 or 99. They should then add the remaining part of the 3-digit number to the result. They may draw number bonds to help them, if necessary. Guidance is provided for Task 1(a).



Exercise 9 Mental Subtraction1. Subtract mentally.

a) 65 – 8 =

b) 50 – 4 =


a) 25 – 7 = b) 64 – 6 =

c) 82 – 4 = d) 42 – 8 =

e) 47 – 8 = f) 73 – 7 =

g) 51 – 5 = h) 87 – 9 =

i) 20 – 2 = j) 60 – 4 =

k) 70 – 5 = l) 50 – 7 =

65 – 8 55 10

50 – 4

10 – 8 =

55 + =

– 4 =

+ =

57

22

1040 6 46

640 10

57

46

18

78

39

46

18

65

58

34

66

78

56

43


Exercise 10 Mental Subtraction1. Fill in the missing numbers.

a) 72

2

b) 83

c)

50 2

d)

40 9


a)

– 25

– 20

68

– 5

b)

– 54

– 50

75

– 4

c)

– 43

– 40

54

– 3

d)

– 76

– 70

98

– 6


a) 74 – 32 = b) 69 – 57 =

c) 87 – 64 = d) 55 – 41 =

e) 46 – 25 = f) 38 – 13 =

3

52 49

70

48

43

14

11

28

22

21

25

42 12

23 14

21 25

80



1 To mentally subtract a 1-digit number from a 2-digit number with regrouping

Students are guided to split each 2-digit number into 10 and

before adding the result to the remaining part of the 2-digit number.

2 To mentally subtract a 1-digit number from a 2-digit number with regrouping

They may draw number bonds to help them.



1 a number bond

Students are to recognize that each 2-digit number is split into tens and ones.

2 To subtract a 2-digit number from another 2-digit number without regrouping

Students are required to follow the direction of the arrows and subtract the given values from the previous 2-digit numbers. They should see that subtracting a 2-digit number from another 2-digit number without regrouping is the same as subtracting the tens and ones of the 2-digit number separately.

3 To mentally subtract a 2-digit number from another 2-digit number without regrouping answers.



Exercise 12 Mental Subtraction1. Subtract mentally.

a) 450 – 8 =

b) 683 – 5 = c) 891 – 3 =

d) 565 – 9 = e) 236 – 8 =

f) 950 – 6 = g) 144 – 7 =


a) 539 – 70 =

b) 748 – 90 = c) 266 – 80 =

d) 353 – 70 = e) 407 – 30 =

f) 625 – 80 = g) 238 – 40 =

450 – 8 440 10

10 – 8 =

+ 440 =

539 – 70 439 100

100 – 70 =

+ 439 =

442

469

888

228

678

556

944 137

186

377

198

658

283

545

22 442

3030 469


Exercise 11 Mental Subtraction1. Subtract mentally. Example 877 – 5 =

a) 938 – 4 = b) 415 – 3 =

c) 269 – 7 = d) 104 – 1 =

e) 655 – 2 = f) 788 – 6 =

2. Subtract mentally. Example 583 – 80 =

a) 767 – 10 = b) 161 – 40 =

c) 357 – 30 = d) 280 – 50 =

e) 876 – 70 = f) 692 – 60 =

3. Subtract mentally. Example 400 – 300 =

a) 700 – 500 = b) 900 – 200 =

c) 735 – 500 = d) 627 – 400 =

e) 486 – 200 = f) 768 – 600 =

872

503

100

7 ones – 5 ones = ones2

8 tens – 8 tens = tens0

4 hundreds – 3 hundreds = hundred1

412

103

934

262

653 782

121

230

632

757

327

806

700

235

286

200

227

168



1 To mentally subtract ones from a 3-digit number without regrouping

Students should recognize that when they subtract ones from a 3-digit number without regrouping, only the number of ones in the 3-digit number changes. An example is provided to guide them.

2 To mentally subtract tens from a 3-digit number without regrouping

Students should recognize that when they subtract tens from a 3-digit number without regrouping, only the number of tens in the 3-digit number changes. An example is provided to guide them.

3 To mentally subtract hundreds from a 3-digit number without regrouping

Students should see that only when they subtract hundreds from a 3-digit number without regrouping, only the number of hundreds in the 3-digit number changes. An example is provided to guide them.



1 To mentally subtract ones from a 3-digit number with regrouping

Students are to split the 3-digit number into 10 and another number, so that they can subtract the ones from 10 before adding the remaining part of the 3-digit number to the result. Guidance is provided for Task 1(a).

2 To mentally subtract tens from a 3-digit number with regrouping

Students are to split the 3-digit number into 100 and another number, so that they can subtract the tens from 100 before adding the remaining part of the 3-digit number to the result. Guidance is provided for Task 2(a).



Exercise 13 Mental Subtraction


a) 300 – 99 =

b) 500 – 99 = c) 700 – 99 =

d) 800 – 99 = e) 400 – 98 =

f) 600 – 98 = g) 900 – 98 =


a) 180 – 99 =

b) 302 – 99 = c) 556 – 99 =

d) 848 – 99 = e) 205 – 98 =

f) 467 – 98 = g) 632 – 98 =

300 – 99 200 100

100 – 99 =

+ 200 =

180 – 99 80 100

100 – 99 =

+ 80 =

201

81

601

302

401

701

502 802

457

107

203

749

369 534

12011

11 81


215 + 700

368 – 20

106

136 – 30

423 + 50

948 – 9578 – 300

Example

3. Add or subtract mentally.

a)

b) c)

d) e)


+ 22

– 31 – 12

+ 40

43

67

– 34

+ 15

89

55

58

98

915

473348

939278

Practice Book Exercise 12 (continued)


3 To mentally add or subtract ones, tens or hundreds to or from a 3-digit number

Each part is presented as a conversation. Teachers may allow students to work in pairs and model the conversation. Task 3(d) involves regrouping. An example is provided to guide students.

4 To mentally add or subtract a 2-digit number to or from another 2-digit number without regrouping

number, before adding or subtracting the ones.



1 To mentally subtract 98 or 99 from hundreds

Students are to split the hundreds into 100 and another number so as to subtract 98 or 99 from 100. They should then add the remaining part of the hundreds to the result. Guidance is provided for Task 1(a).

2 To mentally subtract 98 or 99 from a 3-digit number

Students are to split the 3-digit number into 100 and another number so as to subtract 98 or 99 from 100, and then add the remaining part of the 3-digit number to the result. Guidance is provided for Task 2(a).

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TR9.2 Mental Addition Strategies Mind Map

Addition

Without regrouping

With regrouping

2-digit + 2-digit

Split one 2-digit number into tens and ones. Add the tens to the other number, then add the ones.

3-digit +

ones

tens

hundreds

2-digit + 1-digit

3-digit +

ones

tens

1-digit + 98 or 99

2-digit + 98 or 99

3-digit + 98 or 99

Add the ones to the ones of the 3-digit number. The number of hundreds and tens in the 3-digit number do not change.

Add the tens to the tens of the 3-digit number. The number of hundreds and ones in the 3-digit number do not change.

Add the hundreds to the hundreds of the 3-digit number. The number of tens and ones in the 3-digit number do not change.

Make a ten with the ones in the 2-digit number, then add the remaining part of the 1-digit number to the result.

Make a ten with the ones in the 3-digit number, then add the remaining part of the ones to the result.

Make 100 with the tens, then add the remaining part of the 3-digit number to 100.

Make 100 with 98 or 99, then add the remaining part of the 1-digit number to the result.



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TR9.3 Mental Subtraction Strategies Mind Map

Split the smaller 2-digit number into tens and ones. Subtract the tens and the ones separately.

Subtract the ones from the ones of the 3-digit number. The number of hundreds and tens in the 3-digit number do not change.

Subtract the tens from the tens of the 3-digit number. The number of hundreds and ones in the 3-digit number do not change.

Subtract the hundreds from the hundreds of the 3-digit number. The number of tens and ones in the 3-digit number do not change.

Split the 2-digit number into 10 and another number. Subtract the 1-digit number from 10, then add the remaining part of the 2-digit number to the result.

Split the 3-digit number into 10 and another number. Subtract the ones from 10, then add the remaining part of the 3-digit number to the result.

Split the 3-digit number into 100 and another number. Subtract the tens from 100, then add the remaining part of the 3-digit number to the result.

Split the hundreds into 100 and another number. Subtract 98 or 99 from 100, then add the remaining part of the hundreds to the result.

Split the 3-digit number into 100 and another number. Subtract 98 or 99 from 100, then add the remaining part of the 3-digit number to the result.

Subtraction

Without regrouping

2-digit – 2-digit

2-digit – 1-digit

3-digit –

ones

tens

hundreds

With regrouping

3-digit –

ones

tens

hundreds – 98 or 99

3-digit – 98 or 99

singapore, republic of korea and hong kong teacher’s guide€¦ · with regrouping • to...

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