Introduction

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Think It Through

Lesson 3 Understand Mental Math Strategies (Make a Ten)

Lesson 3Understand Mental Math Strategies (Make a Ten)

You know how to break apart numbers into tens and ones.

You can break apart numbers to make a 10 when you add or subtract in your head.

Add 9 1 7.

Think of 9 red counters and 7 blue counters on two 10-frames.

The first blue counter There are 6 more blue counters makes a full 10. in the second 10-frame.

Add 9 1 7. Think of 7 as 1 1 6. Add 9 and 1 to make 10.

Write the sums.

10 1 6 5 9 1 7 5

10 1 7 5 17

10 7

17

10 1 9 5 19

10 9

19

Think You can make a 10 to add.

9 1 7 9 1 1 1 6 10 1 6

How can you make a 10 to add and subtract in your head?

2.OA.B.2

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Subtract 14 2 6 .

Start with 14 counters.

Put an X over the counters in the second 10-frame. How

many counters do you subtract to get to 10?

How many more counters need an X to subtract a

total of 6? Put an X over this number of counters in the first 10-frame.

Use the 10-frames to complete each equation.

14 2 5 10

10 2 5 8

Subtracting 6 is the same as subtracting 4 and then subtracting 2 more. So, 14 2 6 5 8.

Reflect Work with a partner.

1 Talk About It How can you make a 10 to help you add 8 1 7?

Write About It

Think You can make a 10 to subtract.

When I have 14, I have 10 and 4 more.

Guided Instruction

Think About

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Lesson 3

Lesson 3 Understand Mental Math Strategies (Make a Ten)

The Strategy of Making a Ten

Answer Problems 2 through 4 to help you think about 8 1 8 5 16.

2 Complete the equations.

8 1 5 10 10 1 5 16

3 Use your answers from Problem 2 to fill in the boxes on the open number line.

8 10 16

4 Complete the equation. 8 1 8 5 10 1

Answer Problems 5 through 7 to help you think about 13 2 7 5 6.

5 Fill in the boxes.

6 10 13

6 Use the open number line to complete the equations.

13 2 5 10 10 2 5 6

7 Complete the equation. 13 2 5 6

Let’s Explore the Idea Use open number lines to add and subtract.

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Let’s Talk About It Work with a partner.

8 Look at the open number line in Problem 3. Why do you add 2 to 8?

Why do you add 6 next?

9 Look at the open number line in Problem 5. Why do you first subtract 3 from 13?

Why do you subtract 4 next?

Try It Another Way Use a ladder model.

Fill in the blanks to add or subtract.

10 Find 8 1 6 by adding up.

start

? 8 1 6 5

10 10 1 5

8 8 1 510

11 Find 14 2 6 by subtracting down.

start14 14 2 510

10 10 2 5

? 14 2 6 5

Guided Practice

Connect

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Lesson 3

Lesson 3 Understand Mental Math Strategies (Make a Ten)

Ideas About Making a Ten

Talk about these questions as a class. Then write your answers.

12 Demonstrate Show how to make a 10 to find 6 1 9. Explain your thinking.

13 Compare Greta and Chuck each added 5 1 7 by making a 10.

Greta 5 1 7Break apart 5. 2 1 3 7 1 3 5 10Add 2 more. 10 1 2 5 12 7 1 5 5 12

Chuck 5 1 7Break apart 7. 5 1 2 5 1 5 5 10Add 2 more. 10 1 2 5 12 7 1 5 5 12

Did they both make a 10 correctly? Explain.

14 Analyze Ming added 9 1 8. See her work at the right. What did she do wrong? What is the correct answer?

9 1 1 5 10 10 1 8 5 18 9 1 8 5 18

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Independent Practice

Apply

Lesson 3

Lesson 3 Understand Mental Math Strategies (Make a Ten)

Ideas About Making a Ten

Put It Together Use what you have learned to complete this task.

15 Think about making a 10 to add or subtract.

Part A Choose two numbers you can add by making a 10. Write an addition equation using your numbers.

Draw a model to show how to answer your addition problem by making a 10. Then solve your addition problem.

Part B Choose two numbers you can subtract by making a 10. Write a subtraction equation using your numbers.

Draw a model you could use to solve your subtraction equation by making a 10. Then solve your subtraction problem.

Part C How can making a 10 help you add or subtract in your head?

1 5 ?

1 5

2 5 ?

2 5