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6.8: Differences of Fractions with Different Denominators

Difficulty Level: At Grade Created by: CK-12
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Let's Think About It

Cindy is painting her room. She has \begin{align*} \frac{6}{8}\end{align*} of a gallon of paint at the start. She used \begin{align*} \frac{1}{2}\end{align*} of a gallon to paint one wall. How much paint does Cindy have left? Will she have enough to paint another wall? 

In this concept, you will learn how to subtract fractions with different denominators.

Guidance

You can add fractions with different denominators by rewriting the fractions with a common denominator before adding. The same step is taken to subtract fractions with different denominators. 

To find the difference of two fractions with different denominators, rewrite the fractions with a common denominator before subtracting. 

Here is a subtraction problem.

\begin{align*}\frac{6}{8} - \frac{1}{4} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

The fractions in this problem have different denominators, 8 and 4. 

Rewrite the fractions so they share a common denominator. The least common denominator (LCD) is the lowest common multiple (LCM) of 8 and 4.

8: 8, 16, 24 . . .

4: 4, 8, 12, 16 . . .

The LCM is 8. Find the equivalent of each fraction with the denominator 8.  \begin{align*}\frac{6}{8}\end{align*} is already in terms of eighths. Multiply the numerator and denominator of \begin{align*}\frac{1}{4}\end{align*} by  2. 

\begin{align*}\frac{1 \times 2}{4 \times 2} = \frac{2}{8}\end{align*}

\begin{align*}\frac{6}{8} - \frac{1}{4} =\frac{6}{8} - \frac{2}{8} \end{align*}

Now you can subtract the fractions with the common denominator. Subtract the numerators over the common denominator.

\begin{align*}\frac{6}{8} - \frac{2}{8} = \frac{4}{8}\end{align*}

Finally, simplify the fraction by dividing the numerator and the denominator by the greatest common factor (GCF). The GCF of 4 and 8 is 4. Divide the numerator and the denominator by 4.

\begin{align*}\frac{4 \div 4}{8 \div 4} = \frac{1}{2}\end{align*}

The difference is \begin{align*}\frac{1}{2}\end{align*}.

Guided Practice

Find the difference.

\begin{align*}\frac{3}{4} - \frac{6}{12} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

The fractions do not have a common denominator. The denominators are 4 and 12. 

First, find the LCM of 4 and 12. The LCM is 12. 

4: 4, 8, 12, . . .

12: 12, 24 . . .

Then, rewrite the fractions with the common denominator of 12. 

 \begin{align*}\frac{3}{4}=\frac{9}{12}\end{align*}

 \begin{align*}\frac{3}{4} - \frac{6}{12} = \frac{9}{12} - \frac{6}{12}\end{align*} 

Next, subtract the fractions. Subtract the numerators over the common denominator. 

\begin{align*}\frac{9}{12} - \frac{6}{12} = \frac{3}{12} \end{align*}

Finally, simplify the fraction.

 \begin{align*}\frac{3}{12} = \frac{1}{4}\end{align*}

The difference is \begin{align*}\frac{1}{4}\end{align*}.

Examples

Subtract the following fractions. Answer in simplest form.

Example 1

\begin{align*}\frac{5}{6} - \frac{1}{3} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

First, find the LCM of 6 and 3. The LCM is 6.

Then, rewrite the fractions with a common denominator. 

 \begin{align*}\frac{1}{3} = \frac{2}{6}\end{align*}

 \begin{align*}\frac{1}{2}\end{align*}

Next, subtract the fractions. 

 \begin{align*}\frac{5}{6} - \frac{2}{6}=\frac{3}{6}\end{align*}

Finally, simplify the fraction.

 \begin{align*}\frac{3}{6}=\frac{1}{2}\end{align*}

The difference is \begin{align*}\frac{1}{2}\end{align*}.

Example 2

\begin{align*}\frac{1}{2} - \frac{4}{9} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

First, find the LCM of 2 and 9. The LCM is 18.

Then, rewrite the fractions with a common denominator. 

 \begin{align*}\frac{1}{2} &= \frac {9}{18} \\ \frac{4}{9} &= \frac {8}{18}\end{align*}

 \begin{align*}\frac{1}{2} - \frac{4}{9} = \frac {9}{18} - \frac {8}{18}\end{align*}

Next, subtract the fractions. 

 \begin{align*}\frac {9}{18} - \frac {8}{18} = \frac{1}{18}\end{align*}

The fraction is in simplest form.

The difference is \begin{align*}\frac{1}{18}\end{align*}.

Example 3

\begin{align*}\frac{4}{5} - \frac{1}{4} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

First, find the LCM of 5 and 4. The LCM is 20.

Then, rewrite the fractions with a common denominator. 

 \begin{align*}\frac{4}{5} & = \frac {16}{20} \\ \frac{1}{4} & = \frac{5}{20}\end{align*}

 \begin{align*} \frac{4}{5} - \frac{1}{4} = \frac {16}{20} - \frac{5}{20}\end{align*}

Next, subtract the fractions. 

 \begin{align*} \frac {16}{20} - \frac{5}{20} = \frac{11}{20}\end{align*}

The fraction is in simplest form.

The difference is \begin{align*}\frac{11}{20}\end{align*}.

Follow Up

Remember Cindy and her paint project?

Cindy had \begin{align*} \frac{6}{8}\end{align*} of a gallon of paint and used \begin{align*} \frac{1}{2}\end{align*} gallon to paint one wall. Subtract the amount used from the original amount to find the amount of paint Cindy has left.

\begin{align*}\frac{6}{8} - \frac{1}{2} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

First, find the LCM of 8 and 2. The LCM is 8.

Then, rewrite the fractions with the common denominator 8.

 \begin{align*}\frac{1}{2} = \frac {4}{8}\end{align*}

 \begin{align*}\frac{6}{8} - \frac{1}{2} = \frac{6}{8} - \frac{4}{8}\end{align*}

Next, subtract the fractions.

\begin{align*}\frac{6}{8} - \frac{4}{8} =\frac{2}{8}\end{align*}

Finally, simplify the fraction.

 \begin{align*}\frac{2}{8}=\frac{1}{4}\end{align*}

Cindy has \begin{align*}\frac{1}{4}\end{align*} of a gallon left over. She will not have enough to paint another wall.

Video Review

Explore More

Subtract the following fractions. Answer in simplest form.

1. \begin{align*}\frac{4}{8} - \frac{1}{8} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

2. \begin{align*}\frac{9}{10} - \frac{1}{2} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

3. \begin{align*}\frac{10}{10} - \frac{1}{3} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

4. \begin{align*}\frac{15}{16} - \frac{2}{8} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

5. \begin{align*}\frac{9}{10} - \frac{1}{3} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

6. \begin{align*}\frac{3}{5} - \frac{1}{3} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

7. \begin{align*}\frac{9}{10} - \frac{1}{4} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

8. \begin{align*}\frac{20}{30} - \frac{1}{5} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

9. \begin{align*}\frac{18}{19} - \frac{2}{19} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

10. \begin{align*}\frac{4}{6} - \frac{1}{8} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

11. \begin{align*}\frac{7}{8} - \frac{4}{9} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

12. \begin{align*}\frac{1}{2} - \frac{1}{3} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

13. \begin{align*}\frac{4}{5} - \frac{1}{3} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

14. \begin{align*}\frac{7}{9} - \frac{2}{5} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

15. \begin{align*}\frac{11}{12} - \frac{2}{3} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

16. \begin{align*}\frac{6}{7} - \frac{4}{5} = \underline{\;\;\;\;\;\;\;\;\;}\end{align*}

Answers for Explore More Problems

To view the Explore More answers, open this PDF file and look for section 6.8. 

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Vocabulary

TermDefinition
Least Common Multiple The least common multiple of two numbers is the smallest number that is a multiple of both of the original numbers.
Lowest Common Denominator The lowest common denominator of multiple fractions is the least common multiple of all of the related denominators.
Renaming fractions Renaming fractions means rewriting fractions with different denominators, but not changing the value of the fraction.

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