multiplication and division of fractions and decimals session 3 january 15, 2013 huntersville...
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Multiplication and Division of Fractions and
Decimals
Session 3
January 15, 2013Huntersville Elementary
Solve……….
A number with 3 digits after the decimal point is rounded to 4.8 when rounded to the nearest tenth.
What is the smallest this number could be?
What is the largest this number could be?
Fraction Representations Pizzas, pies, and candy bars
Number lines/Sentence strips
Fraction bars/Centimeter grid paper
Clocks
Percent strips- percent and decimal equivalents
Arrays (4 x 6 and 5 x 12)
Benchmarks (containers)
Decomposing fractions 7/8 = 3/6 + 1/4 + 1/8 (Fraction Track Game)
Pattern blocks (unit fractions)
Investigations’ Bar diagrams
Paper folding and Open arrays
Equations
To multiply unit fractions, multiply the denominator times the denominator and
To multiply any two fractions, multiply the numerator times the numerator, and the denominator times the denominator
When you multiply a whole number times a fraction less than 1, the answer is smaller than the whole number
When you multiply a whole number times a fraction greater than 1, the answer is greater
than the whole number
Fraction Conjectures
Fraction Conjectures continued
When you multiply two fractions that are both less than 1, the product is a fraction smaller than either of the factors
When you divide a whole number by a fraction less than 1, the answer is larger than the whole number
When you divide a fraction by a whole number, the answer is smaller than the whole number and the fraction
10 ÷ ½ = 10 x 2
10 x ½ = 10 ÷ 2
The cafeteria made lunches for the fifth graders going on a field trip. They were in four different groups so the number of sandwiches differed. The sandwiches were all the same size.
Group One had 4 students sharing 3 subsGroup Two had 5 students sharing 4 subsGroup Three had 8 students sharing 7 subsGroup Four had 5 students sharing 3 subsDid each student get a “fair share?”If not, which group ate the least?
Most? How do you know?
Sharing Submarine Sandwiches
Next trip we want to guarantee that each student will receive 2/3 of a sub
Using large paper, create a chart for the cafeteria to help them know how many subs to make for up to 15 students
What patterns do you notice? What strategy could cafeteria workers use
for any number of students? If you knew there were 8 subs made, how
could you figure out how many students could each get 2/3 sub?
Model this situation using numbers and symbols.
Help the Cafeteria Staff
Dividing a Whole Number by a Fraction
What does 6 ÷ ½ mean?
How does this problem relate to multiplication?
How is multiplication and division of whole numbers connected to multiplication and division of fractions?
What is meant by “there are two wholes when dividing fractions?” Give an example..
Using the number line below, label the point
1.68
Rounding Decimals
1.6 1.7
How did you know where to place the number?
Procedure for Rounding Underline the rounding place Circle the digit to the right If the circled digit is 5 or greater, increase
the underlined digit If the circled digit is less than 5, leave the
underlined digit as it is Drop the digits to the right of the underlined
digit 4.923
Round to the nearest hundredthWhat about 2.97 rounded to the nearest
tenth?
Write the following number using expanded notation: 689,738
Use exponents when possible
What number is 5,000 less than this?What number is 200 more?
Expanded Notation
Note the suggestions in the snap-ins!
Mercedes had 1.86 grams of gold. She used 0.73 grams of it in a piece of jewelry. How much gold does she have left?
Use of Hundredths Grids Subtract in parts Add up from 0.73 to 1, and then from 1 to 1 and 86
hundredths
Decimal Subtraction Problems
Multiplying Powers of 10 Use 4 Hundredths grids to show the
following 4 x 0.01
4 x 0.1How can we use what we know about multiplying
fractions to help us solve the two problems above?
4 x 1 = 4 x 0.1 =4 x 10 = 4 x 0.01 =4 x 100 = 4 x 0.001 = ?
What relationships do you notice?
25 x 0.01 = 0.25 25 x 0.1 = 2.5 25 x 1 = 25 25 x 10 = 250 25 x 100 = 2,500
Multiplying by “Small” Numbers
2 x 7 = 2 x 0.7 =
Use a number line from 0 to 2 to show the answer to the second problem
Use the same number line to show the answer to the following: 2 x 0.07
(notice the use of running context p CC110-111)
32 x 0.8 = 2.56 25.6 256
Reasoning with Decimals
Problem: 185 x 0.4 =
If the answer to 185 x 4 is 740, how can we use reasoning to determine the answer to the above problem?
Writing a Rule for Multiplication of Decimals
Multiply the numbers like they are whole numbers and then think about the size of the factors and place the decimal point so the product is the right size.
Multiply a whole number by a decimal, and the answer has the same number of decimal places as the decimal number being multiplied. Or, if each of the numbers has one decimal place, then the answer has two decimal places.
Therefore solve: 42 x 36 = 1,512 4.2 x 3.6 = ?????
Multiplying Tenths….
0.2 x 0.4 =
Think about one of the conjectures we made about multiplying 2 fractions less than 1
How can this conjecture and what you know about the relationship between decimals and fractions
help you solve this problem?
Dividing Powers of 10
2 ÷ 1 = ?2 ÷ 0.1 = ?
2 ÷ 0.01 = ?
What does 2 ÷ 1 mean?
Will the answer to each of these be greater or less than 2? How do you know?
How can we use 2 hundreds grids to represent these situations?
Comparing Multiplication and Division
25 x 100 = 2,500 25 ÷ 100 = ____25 x 10 = 250 25 ÷ 10 = ____25 x 1 = 25 25 ÷ 1 = 2525 x 0.1 = 2.5 25 ÷ 0.1 = 25025 x 0.01 = 0.25 25 ÷ 0.01 = 2,500
What patterns do you notice?How can you use your understanding of fractions to
figure out the answers to the two unsolved problems?
Bonus: use the pattern in the division problems to figure out the answer to 25 ÷ 0.001
Closest Estimate6.8 x 2.3 ≈ 1.4 14 14074 x 8.1 ≈ 5.6 56 560166 x 0.08 ≈ 1.66 16.6 166
What is the closest estimate for each?
Is the closest estimate greater or less than the actual answer?
How do you know?
Dividing Decimals
18 ÷ 6 = 18 ÷ 0.6 =How could you write these problems as
missing factors problems?
Draw a number line from 0-20Solve each of these using the number line.
What are some rules for dividing decimals?
Problems can be solved by thinking about both numbers as whole numbers and then reasoning about where the decimal belongs
Problems can be solved by thinking about the division problem as a missing factor problem to solve or to check the reasonableness of the answer
Will the rule of counting decimal places work for division like it does for multiplication?
97.5÷ 6.5 =
21.52 ÷ 0.8 =
www.illustrativemathematics.org
DPI Fraction UnitTest-item bankCMS WikiUnpacking Document
Resources
Why don’t we just teach them the rule?
There are easy rules for multiplying and dividing fractions and decimals. So…..
Reflection……
Thinking about multiplying and dividing fractions, why do you think it has been said that “fractions are the pathway from arithmetic to algebra?
Is multiplying or dividing fractions more difficult? Explain your reasoning.
Is multiplying or dividing decimals more difficult? Explain your reasoning