Sign In
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.
Show less Show more expand_more| Student Learning Objectives: |
|---|
|
| | 10 Theory slides |
| | 9 Exercises - Grade E - A |
| | Each lesson is meant to take 1-2 classroom sessions |
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.
What fraction of the cake did he eat?
What fraction of the cake is left for others to eat?
Fractions can be classified based on whether or not they have the same denominator.
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.
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.
Determine whether the given fractions are like fractions or unlike 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.
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
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.
| 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
a/b=a * 4/b * 4
Multiply
a/b=a * 5/b * 5
Multiply
Subtract fractions
Subtract term
Put minus sign in front of fraction
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.
Dylan takes three berry pie pieces. Emily takes two pieces for herself. What fraction of the berry pie did they eat altogether?
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?
What fraction of the berry pie did Dylan eat? What fraction of the pie did Emily eat?
Use the answer found in Part A to find the total amount of pie eaten.
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.
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.
Split into factors
Cancel out common factors
Simplify quotient
Dylan and Emily ate 12 of the berry pie.
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.
a/b=a * 6/b * 6
Multiply
Add fractions
Add terms
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.
Each slice represents 112 of a pie, so the 11 brown slices make 1112 of a pie eaten.
Perform the indicated operation. Simplify the result if possible.
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?
| 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.
a/b=a * 28/b * 28
a * 1=a
a/b=a * 4/b * 4
Multiply
Now we can evaluate the expression.
Add fractions
Add terms
Subtract fractions
Subtract term
The numerator and denominator of this fraction have a common factor of 7. Let's simplify it.
Split into factors
Cancel out common factors
Simplify quotient
The fraction of the students playing games is 34.
A cake recipe calls for 2 14 cups of flour, 1 23 cups of sugar, and 12 cup of oil.
What is the total amount of the listed ingredients needed to bake the cake? Write the answer as a mixed number.
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.
Start by converting the given mixed numbers into improper fractions.
Subtract 1 23 from 2 13.
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.
a/b=a * 3/b * 3
a/b=a * 4/b * 4
a/b=a * 6/b * 6
Multiply
Add fractions
Add terms
Finally, let's write our fraction as a mixed number.
Write as a sum
Write as a sum of fractions
Calculate quotient
Add terms
A total of 4 512 cups of ingredients are needed to bake the cake.
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.
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.
An extra 23 cup of sugar was used.
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.
What fraction of the cake did he eat?
What fraction of the cake is left for others to eat?
Add the given fractions.
Subtract the Part A's answer from 1.
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.
a/b=a * 3/b * 3
a/b=a * 2/b * 2
Multiply
Add fractions
Add terms
Dylan ate 730 of the cake.
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.
Rewrite 1 as 30/30
Subtract fractions
Subtract term
Dylan ate 730 of the cake, so 2330 of the cake remains for his guests.
Let's compare the denominators of the fractions. 4/14 and 4/24 We see that one fraction has a denominator of 14 and the other has a denominator of 24. The fractions do not have the same denominator. Therefore, we can say that they are unlike fractions.
Let's first determine which fractions the diagrams represent. Each diagram consists of 10 parts. The number of shaded parts is 3 for the diagram on the left and 7 for the other diagram.
The diagram on the left with three shaded parts represents 310. The diagram on the right with seven shaded regions parts 710. Both fractions have the same denominator. Therefore, they are like fractions.
We see that both fractions have a denominator of 12. 7/12 + 5/12 The fractions are like fractions. The sum of the fractions is the sum of the numerators divided by 12.
We can simplify this fraction to 1 because the numerator and denominator are the same. 12/12 = 1
We want to find the difference of two fractions that have the same denominator.
15/16-3/16
We subtract the numerators to find this difference.
The numerator and denominator have a common factor, which is 4. We can simplify the fraction. Let's rewrite the numerator and denominator as a product of its factors. Then we will cancel out the common factors.
The difference of the fractions is 34.
We see that the fractions have different denominators. 11/12 - 5/8 To subtract fractions with different denominators, we need to rewrite the fractions as equivalent fractions with the least common denominator. We need to determine the prime factors of the denominators to find the least common denominator.
| Denominators | Prime Factorization |
|---|---|
| 12 | 2^2 * 3 |
| 8 | 2^3 |
The least common denominator is the product of the highest power of each factor. Least Common Denominator 2^3 * 3 = 24 We can now rewrite the denominators. We will multiply the numerator and the denominator of the first fraction by 2. Then, we will multiply the numerator and the denominator of the second fraction by 3. Once the fractions have the same denominator, we can subtract them.
The difference of the fractions is 724.
We want to find the sum of two fractions that are unlike fractions.
7/8+1/2
To add fractions with different denominators, we need to rewrite the fractions as equivalent fractions with the least common denominator. To do so, let's first determine the prime factors of the denominators.
| Denominators | Prime Factorization |
|---|---|
| 8 | 2^3 |
| 2 | 2 |
The least common denominator is the product of the highest power of each factor. Least Common Denominator 2^3 = 8 We see that the first fraction already has this as its denominator. We need to rewrite the second fraction. To do so, we will multiply the numerator and the denominator of it by 4. Once the fractions have the same denominator, we can add them.
The sum of the fractions is an improper fraction, 118. We can write it as a mixed number.
We want to evaluate the sum of the mixed numbers. - 4 34 + ( - 3 14 ) We will start by rewriting the mixed numbers as improper fractions.
| - a bc | - (a* c+b/c ) | Simplify |
|---|---|---|
| - 4 34 | - (4* 4+3/4 ) | - 19/4 |
| - 3 14 | - (3* 4+1/4 ) | - 13/4 |
The mixed numbers are converted into improper fractions. - 4 34 + ( - 3 14 ) ⇕ - 19/4 + (- 13/4 ) Fractions should have the same denominator when we add or subtract them. In this case, the fractions have the same denominators. That means the sum of the fractions is the sum of the numerators divided by 4.
The sum of the mixed number is - 8.
We want to find the given difference.
3 15 - 5 13
Again, we will start by rewriting the mixed numbers as improper fractions.
| a bc | a* c+b/c | Simplify |
|---|---|---|
| 3 15 | 3* 5+1/5 | 16/5 |
| 5 13 | 5* 3+1/3 | 16/3 |
The mixed numbers are converted into improper fractions. 3 15 - 5 13 ⇕ 16/5 - 16/3 Fractions should have the same denominator when they go through addition or subtraction. In this expression, the fractions have different denominators. We multiply the numerator and denominator of each fraction by the denominator of the other to make them like fractions. Let's do it.
We will rewrite the numerator as 30 plus 2 with the aim of writing the fraction as a mixed number.
Evaluate the expression. (- 3/8 + 2/11 ) + 11/8
We see that the expression contains two like fractions, - 38 and 118. Let's write these fractions next to each other. We will use the Commutative Property of Addition to do this. Also, we can change the order of the fractions in the parentheses by the same property.
Next, we can group the last two fractions by the Associative Property of Addition.
We can now add the like fractions.
The value of the expression is 1 211.