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1. Add and Subtract Fractions
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Chapter 3
1. 

Add and Subtract Fractions

This lesson delves into the intricacies of adding and subtracting fractions, focusing on both like and unlike fractions. It starts with real-world scenarios, such as Dylan eating a fraction of a berry pie, to make the concepts relatable. The lesson explains that fractions with the same denominator are called 'like fractions,' while those with different denominators are termed 'unlike fractions.' It then guides you through the process of converting unlike fractions into like fractions so that they can be added or subtracted easily. The lesson also touches on the importance of finding the least common denominator (LCD) when dealing with unlike fractions. It provides step-by-step methods for performing these operations, even when mixed numbers are involved. The aim is to make you comfortable with fraction operations through practical examples and straightforward explanations.

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Student Learning Objectives:
  • Add and subtract like and unlike fractions using LCD and using multiplication
  • Add and subtract mixed numbers
10 Theory slides
9 Exercises - Grade E - A
Each lesson is meant to take 1-2 classroom sessions
Add and Subtract Fractions
Slide of 10
This lesson covers fractions with the same and different denominators and how to add and subtract them.

Catch-Up and Review

Here are a few recommended readings before getting started with this lesson.

Challenge

What Fraction of the Cake Did Dylan Eat?

Dylan's mom baked a vanilla-strawberry cake for his birthday party. Dylan eats one-tenth of the cake before the party even starts. He also eats two-fifteenths of the cake during the party.

a

What fraction of the cake did he eat?

b

What fraction of the cake is left for others to eat?

Discussion

Like and Unlike Fractions

Fractions can be classified based on whether or not they have the same denominator.

Like Fractions

Two or more fractions that have the same denominator are called like fractions. For example, 38, 58, and 68 are all like fractions whose denominators are 8.

Integers such as 6, 11, and 43 are like fractions because they are considered to have a denominator of 1. Their fraction forms are 61, 111, and 431, respectively.

Unlike Fractions

Fractions with different denominators are called unlike fractions. For example, 28, 23, and 35 are unlike fractions because they have different denominators.

Fractions like 13, 26, and 39 are also unlike fractions, even though they all are equivalent to 13. This is because their denominators are different.

The diagram shows pies representing equivalent fractions. The fractions have the same value, but they are unlike fractions because the pies are divided into different numbers of pieces.
Pop Quiz

Identifying Like and Unlike Fractions

Determine whether the given fractions are like fractions or unlike fractions.

Randomly generated fractions
Discussion

Adding and Subtracting Fractions

The first step in adding and subtracting fractions is to check if they share the same denominator. Here, the methods of performing these operations on fractions and how to convert unlike fractions to like fractions will be discussed with examples.

Adding and Subtracting Like Fractions

To add or subtract like fractions, add or subtract the numerators and keep the denominator the same.

a/c + b/c=a + b/c [1.3em] a/c - b/c=a - b/c

Adding and Subtracting Unlike Fractions

Unlike fractions must first be converted to like fractions when adding or subtracting. One way to convert them is to multiply the numerator and denominator of each fraction by the denominator of the other. Then, the given operation can be performed.

a/c + b/d=ad + bc/cd [1.3em] a/c - b/d=ad - bc/cd

Another way is to find the least common denominator (LCD) of the fractions. Consider the example of subtracting 712 from 415. 4/15 - 7/12 The result can be found in four steps.

1
Find the Least Common Denominator
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The least common denominator is the least common multiple of the numbers in the denominators. 4/15 - 7/12 The numbers need to be expressed as a product of their prime factors to find their LCM.

Denominator Prime Factorization
15 3 * 5
12 2^2 * 3

The least common denominator is the product of the highest power of each prime factor. LCD: 2^2 * 3 * 5 = 60

2
Rewrite Each Fraction Using the LCD
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The LCD is 60. Now the fractions will be multiplied by the appropriate factor to make their denominators equal to 60. The first fraction must be multiplied by 4 and the second fraction by 5 to get the LCD.

4/15 - 7/12
4 * 4/15 * 4- 7/12
16/60 - 7/12
16/60 - 7 * 5/12 * 5
16/60 - 35/60

3
Combine the Numerators
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The numbers in the numerators can now be subtracted because both fractions have the same denominator.

16/60 - 35/60
16-35/60
- 19/60
- 19/60

4
Simplify if Possible
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Check if the resulting fraction can be simplified or not. Our fraction cannot be simplified because the numerator and denominator do not have any common factors. - 19/60

Use a similar process to add unlike fractions.
Example

A Piece of Pie

A berry pie and a peanut butter pie are served at a dinner party. The berry pie is cut into 10 equal pieces and the peanut butter pie is cut into 12 equal pieces.

a

Dylan takes three berry pie pieces. Emily takes two pieces for herself. What fraction of the berry pie did they eat altogether?

b

The party guests ate five of the peanut butter pie pieces. What fraction of the total amount of pie, both berry and peanut butter, was eaten at the party?

Hint

a

What fraction of the berry pie did Dylan eat? What fraction of the pie did Emily eat?

b

Use the answer found in Part A to find the total amount of pie eaten.

Solution

a

Dylan ate 3 pieces of the berry pie that was cut into 10 pieces, so he ate 310 of the pie. Emily eats another 2 pieces, or 210, of the same pie. The sum of these fractions gives the amount of pie eaten.

Amount of Pie Eaten 3/10 + 2/10 Notice that both of these fractions have a denominator of 10. This denominator represents the whole pie's number of slices. These fractions are like fractions, so to add them, we simply add their numerators and keep the denominator the same.

3/10 + 2/10
3+2/10
5/10

The denominator is a multiple of the numerator. Let's simplify the fraction. We will rewrite 10 as a product of its factors, then cancel out the common factors.

5/10
5/2 * 5
5/2 * 5
1/2

Dylan and Emily ate 12 of the berry pie.

b

The party guests ate 5 pieces of the 12-piece peanut butter pie, so 512 of this pie was eaten. In Part A, we found that half of the berry pie was also eaten. The sum of these two fractions is the total amount of pie that was eaten at the dinner party.

Total Amount of Pie Eaten 1/2 + 5/12 To add these factions, we need to find equivalent fractions with a common denominator. Let's find the prime factors of these numbers to help us find the least common denominator.

Denominators Prime Factorization
2 2
12 2^2 * 3

The least common denominator is 2^2 * 3, or 12. The second fraction already has a denominator of 12, so we will multiply the first fraction's numerator and denominator by 6. Then, let's add the fractions.

1/2+5/12
1 * 6/2 * 6+5/12
6/12+5/12
6+5/12
11/12

The dinner party guests ate 1112 of a pie! We can also explain the process of addition visually. The brown part is equal to 11 of the 112 sized pieces.

Adding fractions

Each slice represents 112 of a pie, so the 11 brown slices make 1112 of a pie eaten.

Pop Quiz

Finding the Sum and Difference of Fractions

Perform the indicated operation. Simplify the result if possible.

Random fractions are shown

Extra

Need an Extra Hand?
We can only add and subtract fractions when they have the same denominators. Once we have a common denominator, combine the numerators and keep the denominator the same.
Example

Students Playing Games

At a school festival, 328 of the students are dancing, while 17 of the students are just listening to the music. The remaining students are playing games. What fraction of students are playing games?

Hint

Subtract the total fraction of the students who are listening to music and dancing from the whole, or 1.

Solution

The given fractions 328 and 17 represent the portion of the students who are dancing and listening to music, respectively. The whole, or 1, represents all students at the festival.

All Students Fraction of the Students Who Are Dancing Fraction of the Students Who Are Listening to Music
1 3/28 1/7

Let's subtract the sum of the two fractions from 1 to find the fraction of students who are playing games. 1 - ( 3/28 + 1/7 ) Every integer has 1 as its denominator. 1/1 - ( 3/28 + 1/7 ) The fractions are unlike fractions because they have different denominators. The fractions must have a common denominator to add or subtract them. Let's find the prime factors of the denominators so we can determine the least common denominator.

Denominator Prime Factorization LCD
1 1 2^2 * 7 =28
28 2^2 * 7
7 7

Next, let's convert the fractions into equivalent fractions with a denominator of 28.

1/1 - ( 3/28 + 1/7 )
Rewrite
1* 28/1 * 28 - ( 3/28 + 1/7 )
28/28 - ( 3/28 + 1/7 )
28/28 - ( 3/28 + 1 * 4/7 * 4 )
28/28 - ( 3/28 + 4/28 )

Now we can evaluate the expression.

28/28 - ( 4/28 + 3/28 )
28/28 - ( 4+3/28)
28/28 - 7/28
28-7/28
21/28

The numerator and denominator of this fraction have a common factor of 7. Let's simplify it.

21/28
3 * 7/4 * 7
3 * 7/4 * 7
3/4

The fraction of the students playing games is 34.

Example

Finding the Cups of Ingredients

A cake recipe calls for 2 14 cups of flour, 1 23 cups of sugar, and 12 cup of oil.

Ingredients.jpg

a

What is the total amount of the listed ingredients needed to bake the cake? Write the answer as a mixed number.

b

Suppose someone uses 2 13 cups of sugar instead. How much extra sugar did they use compared to the original recipe? Simplify the answer if possible.

Hint

a

Start by converting the given mixed numbers into improper fractions.

b

Subtract 1 23 from 2 13.

Solution

a

We want to find the sum of the given mixed numbers and fraction.

2 14 + 1 23 + 1/2 We will start by converting the mixed numbers into improper fractions.

a bc a* c+b/c Simplify
2 14 2* 4+1/4 9/4
1 23 1* 3+2/3 5/3

So, we want to find the value of the following expression. 9/4 + 5/3 + 12 Since the denominators are different, our fractions are unlike fractions. We will convert them to like fractions before adding them. Let's find the least common denominator of the three fractions.

Denominator Prime Factorization LCD
4 2^2 2^2 * 3 = 12
3 3
2 2

We can write the given fractions as equivalent fractions with a denominator of 12. Now let's evaluate the expression.

9/4 + 5/3 + 1/2
Rewrite
9 * 3/4 * 3 + 5/3 + 1/2
9 * 3/4 * 3 + 5* 4/3 * 4 + 1/2
9 * 3/4 * 3 + 5* 4/3 * 4 + 1 * 6/2 * 6
27/12 + 20/12 + 6/12
27+20+6/12
53/12

Finally, let's write our fraction as a mixed number.

53/12
Write fraction as a mixed number
48 + 5/12
48/12+5/12
4+5/12
4 512

A total of 4 512 cups of ingredients are needed to bake the cake.

b

This time someone used 2 13 cups of sugar to bake the cake. That amount is greater than the recipe calls for. Let's rewrite 2 13 as an improper fraction so that we can compare it to 1 23, or 53.

2 13
Write mixed number as a fraction
2 * 3 +1/3
6 +1/3
7/3

The numerator of 73 is greater than the numerator of 53, so 73 is greater than 53. 7/3 > 5/3 We can use these fractions to find how much extra sugar was used. Let's subtract 53 from 73.

7/3 - 5/3
7-5/3
2/3

An extra 23 cup of sugar was used.

Closure

Finding the Amount of Cake in Fractions

Consider the challenge presented at the beginning of the lesson. [-1.1em] Dylan eats $ 110$ of his birthday cake before the party starts. He also eats $ 215$ of the cake during the party.

a

What fraction of the cake did he eat?

b

What fraction of the cake is left for others to eat?

Hint

a

Add the given fractions.

b

Subtract the Part A's answer from 1.

Solution

a

Let's add the given fractions to find the fraction of the cake that Dylan ate. The fractions do not have a common denominator, so we cannot add them immediately. We will find their least common denominator by splitting the denominators into their prime factors.

Denominator Prime Factorization LCD
10 2 * 5 2 * 3 * 5 = 30
15 3 * 5

Let's find equivalent fractions with denominator 30 and add them.

1/10 + 2/15
Rewrite
1 * 3/10 * 3 + 2/15
1 * 3/10 * 3 + 2* 2/15 * 2
3/30 + 4/30
3+4/30
7/30

Dylan ate 730 of the cake.

b

Now we want to find the fraction of the cake that is remaining. From Part A we know that Dylan ate 730 of the cake, so let's subtract 730 from 1. Here, 1 or 3030 represents one whole cake.

1- 7/30
30/30 - 7/30
30-7/30
23/30

Dylan ate 730 of the cake, so 2330 of the cake remains for his guests.



Add and Subtract Fractions
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