2. Multiply Fractions
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Chapter 3
2.
Multiply Fractions
This lesson delves into the methods for multiplying fractions and mixed numbers, as well as simplifying fractions. It covers various techniques, including the use of area models and number lines, to make the concepts more understandable. The lesson also emphasizes the importance of estimation strategies to assess the reasonableness of answers. Practical examples are provided to demonstrate how these mathematical concepts can be applied in everyday situations like calculating time spent on activities or determining the area of objects.
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Lesson Settings & Tools
| | 10 Theory slides |
| | 10 Exercises - Grade E - A |
| | Each lesson is meant to take 1-2 classroom sessions |
Multiply Fractions
Slide of 10
This lesson will cover methods for multiplying fractions by whole numbers, fractions, and mixed numbers. Along with these methods, estimation strategies will be used to assess whether the products are reasonable.
Catch-Up and Review
Here are a few recommended readings before getting started with this lesson.
Challenge
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Running Time
Paulina runs for two-sixths of her free time each day.
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If Paulina has 5 hours of free time per day, how many hours does she run in 4 days? Give the exact answer.
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Multiplying Fractions
The product of two fractions is equal to the product of the numerators divided by the product of the denominators. The resulting fraction is then simplified to its lowest terms, if possible.
a/b * c/d = a * c/b * d
1
Multiply the Numerators
expand_more 2
Multiply the Denominators
expand_more The product of the denominators is 6 * 4 = 24.
3
Simplify if Possible
expand_more Note that 3 is the greatest common factor of 15 and 24. Divide both the numerator and the denominator by 3 to simplify the fraction.
Therefore, the product of 56 and 34 simplified to its lowest terms is 58.
Example
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Finding What Fraction of the Water in a Bottle Is Drunk
Paulina drinks one-third of the water in her bottle before PE class. During class, she drinks ten-twelfths of the remaining water.
a Write a numeric expression to represent the amount of water in the bottle that Paulina drinks during class.
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b Which of the following is the best estimate for the value of the expression written in Part A?
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c Find the value of the expression found in Part A. Simplify the answer if possible.
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Hint
a What fraction of the bottle is full before the class? What mathematical operation must be performed to find ten twelfths of a number?
b If a fraction is greater than or equal to 12, round the fraction to 1. If a fraction is less than 12, round it to 0.
c To multiply fractions, start with the multiplication of the numerators, followed by the multiplication of the denominators. Then, simplify the resulting fractions.
Solution
a Start by finding what fraction of the bottle is full before the physical education class. To do so, subtract one-third from 1.
Notice that each third is divided into six smaller equal parts. The other third can also be divided into six parts. This will make it easier to find what fraction of the whole the red part represents.
The red part shows what fraction of the water bottle Paulina drinks during class.
b Estimation is a great tool to get a rough idea of the result of an operation — in this case, the result of a multiplication. Consider the value of each fraction in the expression separately.
10/12 * 2/3 Now compare each fraction with 12. These fractions do not have the same denominator. The denominators of the fractions are 12, 3, and 2. The least common denominator of these numbers is 12. Each fraction can be rewritten as an equivalent fraction with the common denominator to make it easier to compare them.
| Rewrite | Compare with 12, or 612 | |
|---|---|---|
| 10/12 | 10/12 | 10/12 > 6/12 |
| 2/3 | 2*4/3*4 = 8/12 | 8/12 > 6/12 |
Both fractions are greater than 12. An estimate for the product can then be 1 because the fractions can be rounded to 1. rccc Product: & 10/12 & * & 2/3 [0.5em] & ↓ & & ↓ Estimate: & 1 & * & 1 Since 1 is the identity element of multiplication, this product is also equal to 1. However, that does not say much about the original product, and 1 is not one of the possible answer options. Now think of rounding only one of the fractions to 1 one at a time.
| Estimate for 1012* 23 | |
|---|---|
| Round 1012 to 1 | 1 * 2/3 = 2/3 |
| Round 23 to 1 | 10/12 * 1 = 10/12 |
The fraction 1012 can be simplified to 56. The answer is either 23 or 56. The fractions in the options are in simplest form and 23 is among the options. Therefore, the answer is 23.
c To multiply the fractions, start with the multiplication of the numerators, followed by the multiplication of the denominators. Then, simplify the resulting fraction.
Extra
Using an Area Model to Represent the Product of FractionsIn addition to a number line, the product can also be represented by an area model. In this model, the denominators are used to divide a rectangular diagram into smaller parts. Consider modeling the following product. 10/12 * 2/3 The denominators are 12 and 3. The rectangle is then divided into 12 columns and 3 rows.
The numerators determine which parts will be shaded. Since 10 is the numerator of 1012, 10 of the 12 columns will be shaded. Similarly, 2 of the 3 rows will be shaded.
In this model, the overlapping region represents the product. For this example, the product is 2036= 59.
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Finding the Product of Fractions
The product of two fractions is equal to the product of the numerators divided by the product of the denominators. Practice finding the product of fractions. Simplify the answer to its lowest terms.
Discussion
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Multiplying Fractions by Whole Numbers and By Mixed Numbers
When multiplying fractions by whole numbers or mixed numbers, both factors should be in the form of a proper fraction or an improper fraction.
Multiplying Fractions By Whole Numbers
To multiply fractions with whole numbers, the whole number is written as a fraction with a denominator is 1. The following steps are identical to those for multiplying fractions. Consider multiplying 9 and 227.9 * 2/27
Rewrite
Rewrite 9 as 9/1
9/1 * 2/27
18/27
ReduceFrac
a/b=.a /9./.b /9.
18 /9/27 /9
SimpQuot
Simplify quotient
2/3
Multiplying Fractions By Mixed Numbers
Recall that mixed numbers are fractions that consist of a whole number and a proper fraction. 3 45 whole number: 3 proper fraction: 45 The mixed fraction must be converted into an improper fraction before it can be multiplied by a fraction. The multiplication process can be better understood with the help of an example.3 45 * 1/3
▼
Write mixed number as a fraction
19/5 * 1/3
19/15
19/15
▼
Write mixed number as a fraction
Rewrite
Rewrite 19 as 15+4
15+4/15
WriteSumFrac
Write as a sum of fractions
15/15+4/15
CalcQuot
Calculate quotient
1 + 4/15
Rewrite
Rewrite 1+4/15 as 1 415
1 415
Example
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Finding Times Allocated For Activities
Paulina's PE class lasts for 1 512 hours. The table shows what fraction of the class time is allocated for various activities.
| Activity | Fraction |
|---|---|
| Warm-up | 1/5 |
| Instruction | 1/2 |
| Playing a game | 1/10 |
| Cool-down | 1/5 |
a Find how many minutes are devoted to warming up and cooling down.
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b Find how many minutes are devoted to the activities other than instruction.
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Hint
a Start by determining what fraction of the class time is allocated for warm-up and cool-down. To do so, add the fractions corresponding to the activities. Multiply the portion of the lesson spent on those activities by the total class time. Finally, use the fact that 1 hour is 60 minutes.
b Determine what fraction of the class time is allocated for the activities other than instruction.
Solution
a The fractions for both the warm-up and the cool-down to find the amount of time spent on them. Take a look at the given table.
| Part of 1 512-hour Class | ||
|---|---|---|
| Activity | Fraction | |
| Warm-up | 1/5 | |
| Instruction | 1/2 | |
| Playing game | 1/10 | |
| Cool-down | 1/5 | |
2/5* 1 512
MixedToFrac
a bc=a* c+b/c
2/5* 1 * 12 + 5/12
MultByOne
a * 1=a
2/5* 12 + 5/12
AddTerms
Add terms
2/5* 17/12
60 * 17/30
Rewrite
Rewrite 60 as 60/1
60/1 * 17/30
MultFrac
Multiply fractions
60 * 17/1* 30
Multiply
Multiply
1020/30
CalcQuot
Calculate quotient
34
b Consider the given table again, this time focusing on the instruction time.
| Part of 1 512-hour Class | ||
|---|---|---|
| Activity | Fraction | |
| Warm-up | 1/5 | |
| Instruction | 1/2 | |
| Playing game | 1/10 | |
| Cool-down | 1/5 | |
1/2* 1 512
▼
Write mixed number as a fraction
1/2* 17/12
60 * 17/24
MoveLeftFacToNum
a*b/c= a* b/c
60* 17/24
Multiply
Multiply
1020/24
CalcQuot
Calculate quotient
42.5
Example
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Finding the Area of a Printed Photograph
Paulina loves a photo of her playing volleyball and prints it. The diagram shows the dimensions of the photograph.
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a Estimate the area of the photograph.
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b What is the area of the photograph? Write the answer as a mixed number.
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c Paulina frames the photo using a frame with a 15-centimeter border. What is the area of the photo including the frame? Write the answer as a mixed number.
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Hint
a Use the formula for the area of a rectangle. Round the mixed numbers to the nearest whole numbers
b To multiply fractions, multiply the numerators and denominators with each other.
c Add 2* 15 to each side of the photo. Then repeat the process from Part A.
Solution
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The length of the photo is 8 34 inches and the width of the photo is 6 23 inches. Area = 8 34 * 6 23 Since the fractional parts of the mixed numbers are greater than 12, the mixed numbers can be rounded up. Area = & 8 34 * 6 23 & ↓ ↓ & 9 * 7 Therefore, the area of the photo is about 9* 7, or 63 square inches.
b Recall the expression for the area of the photo from Part A. It is the product of the two mixed numbers.
8 34 * 6 23
MixedToFrac
a bc=a* c+b/c
8* 4 +3/4 * 6 * 3 +2/3
Multiply
Multiply
32+3/4 * 18 +2/3
AddTerms
Add terms
35/4 * 20/3
35/4 * 20/3
MultFrac
Multiply fractions
35* 20/4* 3
Multiply
Multiply
700/12
ReduceFrac
a/b=.a /4./.b /4.
700/4/12/4
CalcQuot
Calculate quotient
175/3
▼
Write fraction as a mixed number
Rewrite
Rewrite 175 as 174+1
174+1/3
WriteSumFrac
Write as a sum of fractions
174/3+1/3
CalcQuot
Calculate quotient
58+1/3
Rewrite
Rewrite 58+1/3 as 58 13
58 13
c In this part, start by adding a 15-inch border on each side of the photo. This will extend each side by 2* 15 inches.
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(8 34+ 2/5) * (6 23+2/5)
▼
Write mixed number as a fraction
MixedToFrac
a bc=a* c+b/c
(8* 4+3/4+ 2/5) * (6 * 3+2/3+2/5)
Multiply
Multiply
(32+3/4+ 2/5) * (18+2/3+2/5)
AddTerms
Add terms
(35/4+ 2/5) * (20/3+2/5)
▼
Add fractions
ExpandFrac
a/b=a * 5/b * 5
(35 * 5/4*5+ 2/5) * (20/3+2/5)
ExpandFrac
a/b=a * 4/b * 4
(35 * 5/4*5+ 2* 4/5* 4) * (20/3+2/5)
ExpandFrac
a/b=a * 5/b * 5
(35 * 5/4*5+ 2* 4/5* 4) * (20*5/3*5+2/5)
ExpandFrac
a/b=a * 3/b * 3
(35 * 5/4*5+ 2* 4/5* 4) * (20*5/3*5+2 * 3/5* 3)
Multiply
Multiply
(175/20+ 8/20) * (100/15+6/15)
AddFrac
Add fractions
175+8/20 * 100+6/15
AddTerms
Add terms
183/20 * 106/15
183/20 * 106/15
MultFrac
Multiply fractions
183* 106/20* 15
Multiply
Multiply
19 398/300
ReduceFrac
a/b=.a /6./.b /6.
19 398/6/300/6
CalcQuot
Calculate quotient
3233/50
▼
Write fraction as a mixed number
Rewrite
Rewrite 3233 as 3200+33
3200+33/50
WriteSumFrac
Write as a sum of fractions
3200/50+33/50
CalcQuot
Calculate quotient
64+33/50
Rewrite
Rewrite 64+33/50 as 64 3350
64 3350
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Finding the Product of Mixed Numbers
To multiply a fraction by a whole number, the whole number is multiplied by the numerator of the fraction. To multiply mixed numbers, the mixed numbers can be converted into improper fractions before multiplying. Practice finding the product of fractions. Simplify the answer to its lowest terms.
Closure
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Finding Time Paulina Runs
The important point in multiplying fractions is to ensure that the fractions are either proper fractions or improper fractions. 1 26 * 2 = 8/6 * 2/1 The final step usually involves simplifying the resulting fraction. However, to make calculations easier, first check if the two fractions are already in their lowest forms. If not, the fractions can be simplified first before multiplying them. 4 & 8/6 &* 2/1 = 4/3 * 2/1 = 8/3 3 & Consider the challenge presented at the beginning of the lesson. Paulina devotes two-sixths of her free time to exercise.
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Hint
Start by finding the number of hours Paulina spends running in a day.
Solution
It is given that Paulina has 5 hours of free time per day and that two-sixths of this time is devoted to physical activities. The time spent on running can be found by finding two-sixths of 5.
2/6 * 5
Notice that the fraction can be simplified to its lowest terms.
Paulina runs for 53 hours every day. Since the number of hours she runs in 4 days is required, the daily amount of activity should be multiplied by 4 to find the total amount of time.
This final fraction can also be written as a mixed number.
In 4 days, Paulina runs 6 23 hours.
20/3
▼
Write fraction as a mixed number
Rewrite
Rewrite 20 as 18+2
18+2/3
WriteSumFrac
Write as a sum of fractions
18/3 + 2/3
CalcQuot
Calculate quotient
6+2/3
Rewrite
Rewrite 6+2/3 as 6 23
6 23
Multiply Fractions
Exercise 2.1
The diagram shows how Zain multiplies two mixed numbers.
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Let's examine the steps Zain followed when multiplying 3 14 and 6 25. We want to figure out if their work is correct.
As we can see, Zain directly multiplies the integer and fraction parts of both mixed numbers with each other. They then add these two products. However, this is an incorrect procedure. We calculate the product of two mixed numbers by rewriting the mixed numbers so that they are improper fractions.
We need to remember that the product of two fractions is equal to the product of the numerators divided by the product of the denominators. Let's find the product!
Finally, let's rewrite the answer as a mixed number and see if our friend was correct.
The result of the product is different from what Zain found. Therefore, Zain is not correct. The answer is C.
We can also calculate the product by using the Distributive Property. Remember, a mixed number a bc is the same as a + bc. Let's rewrite the given expression as follows. 3 14 * 6 25 = (3+1/4) * (6 + 2/5 ) If we apply the Distributive Property here, we get a different expression than the one that Zain found.
There are two missing terms in the Zain's work, 65 and 32. This means that Zain is not correct. Let's continue and find the correct answer.
As we can see, we ended with the same result as before. Therefore, Zain is not correct.
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Ali spends 312 of the day at an amusement park. He spends 49 of that time driving a go-kart.
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We know that Ali spent 312 of his day at an amusement park and 49 of that time driving a go-kart. The product of these two fractions will give us what fraction of the day Ali spent driving the go-kart. Let's find it! 3/12 * 4/9 Notice that we can simplify the first fraction to 14 because 12 is 4 times 3. 3/3* 4 * 4/9 = 1/4* 4/9 We can also simplify across the two fractions. There is a common factor between the numerator of 49 and the denominator of 14. 1/4* 4/9 = 1/1 * 1/9 ⇒ 1/9 Since 1 is the identity element of multiplication, the product is 19. This means that Ali spent 19 of his day driving the go-kart. We can calculate how many hours this ride took by multiplying 19 by the 24 hours in a day.
Ali spent 2 23 hours driving the go-kart.
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Jordan's family owns a paint shop. They have 42 gallon of paint in storage, 59 of which are for outdoor use. The rest of the paint is for indoor use. If each gallon costs $18, what is the total cost of the indoor paint in storage?
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We want to find the total cost of the indoor paint in storage. There are 42 gallons of paint in storage. We know that 59 of that paint is for outdoor use and the rest is for indoor use. Since the total amount of paint is 1 when represented as a fraction, we can determine the fraction of the indoor paint by subtracting 59 from 1.
We found that four-ninths of 42 gallons of paint is for indoor use. 42Liters of Paint ↙ 1.5cm ↘ Outdoor Use Indoor Use 5/9 2cm 4/9 Now we can find the number of gallons of paint for indoor use. To do so, we need to multiply 42 by 49. The result will represent 49 of the 42 gallons of paint in storage. 42 * 4/9 To calculate this product, we will move the whole number to the numerator of the fraction as a factor. Let's do it!
We found that there are 563 gallons of paint for indoor use in storage. Finally, we can find the total cost of the indoor paint in storage. There are 563 gallons paint and each one costs $18. To find the total cost, we need to multiply the fraction and the whole number like we did before.
The total cost of the indoor paint in storage is $336.
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Without calculating the product, complete the statement with <, >, or =.
10/12 * 32/20 10/12
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We want to compare the quantities on each side of the box and complete the statement. 10/12 * 32/20 10/12 Notice that the expression on the left-hand side is 1012 multiplied by a fraction. When we multiply a number by a fraction, the product can be greater than, less than, or equal to the multiplicand, or the original number. If the multiplier fraction is less than 1, the result will be less than the multiplicand. Let's see an example! 1/2 * 1/2 = 1/4 ⇒ 1/4 < 1/2 If the fraction is equal to 1, the result will be equal to the multiplicand. 1/2 * 2/2 = 1/2 ⇒ 1/2 = 1/2 Finally, if the fraction is greater than 1, the result will be greater than the multiplicand. 1/2 * 4/2 = 2 ⇒ 2 > 1/2 Let's focus on the given statement. The fraction 1012 is multiplied by 3220. Since the numerator 32 is greater than the denominator 20, the fraction 3220 is greater than 1. Therefore, the product of 1012 and 3220 is greater than 1012. With this in mind, we can complete the statement. 10/12 * 32/20 > 10/12
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Multiply Fractions
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