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## Arithmetic (all content)

### Course: Arithmetic (all content)>Unit 5

Lesson 9: Common denominators

# Common denominators review

Review finding common denominators, and try some practice problems.

## Common denominators

When fractions have the same denominator, we say they have common denominators.
Having common denominators makes things like comparing, adding, and subtracting fractions easier.

## Finding a common denominator

One way to find a common denominator for two (or more!) fractions is to list the multiples of each denominator until we find the smallest multiple they have in common.
Example
Find a common denominator for start fraction, 7, divided by, 8, end fraction and start fraction, 3, divided by, 10, end fraction.
The denominators are 8 and 10. Let's list multiples of each:
Multiples of 8: 8, comma, 16, comma, 24, comma, 32, comma, start color #11accd, 40, end color #11accd, comma, 48, comma, 56, comma, 64, comma, 72, comma, start color #11accd, 80, end color #11accd, point, point, point
Multiples of 10: 10, comma, 20, comma, 30, comma, start color #11accd, 40, end color #11accd, comma, 50, comma, 60, comma, 70, comma, start color #11accd, 80, end color #11accd, comma, 90, comma, 100, point, point, point
start color #11accd, 40, end color #11accd and start color #11accd, 80, end color #11accd are common multiples of 8 and 10. So, we can use either of these for a common denominator. Most often, we will use the smallest common denominator, so we can work with smaller numbers.
Let's use start color #11accd, 40, end color #11accd for our common denominator.

## Rewriting fractions with a common denominator

Now, we need to rewrite start fraction, 7, divided by, 8, end fraction and start fraction, 3, divided by, 10, end fraction with a denominator of start color #11accd, 40, end color #11accd.
We need to figure out what to multiply each denominator by to get start color #11accd, 40, end color #11accd:
start fraction, 7, divided by, 8, end fraction, times, start fraction, divided by, 5, end fraction, equals, start fraction, divided by, start color #11accd, 40, end color #11accd, end fraction
start fraction, 3, divided by, 10, end fraction, times, start fraction, divided by, 4, end fraction, equals, start fraction, divided by, start color #11accd, 40, end color #11accd, end fraction
Next, we multiply the numerators by the same number as their denominator:
start fraction, 7, divided by, 8, end fraction, times, start fraction, 5, divided by, 5, end fraction, equals, start fraction, 35, divided by, start color #11accd, 40, end color #11accd, end fraction
start fraction, 3, divided by, 10, end fraction, times, start fraction, 4, divided by, 4, end fraction, equals, start fraction, 12, divided by, start color #11accd, 40, end color #11accd, end fraction
Now we have written start fraction, 7, divided by, 8, end fraction and start fraction, 3, divided by, 10, end fraction with a common denominator:
start fraction, 7, divided by, 8, end fraction, equals, start fraction, 35, divided by, 40, end fraction
start fraction, 3, divided by, 10, end fraction, equals, start fraction, 12, divided by, 40, end fraction
Note: The new fractions are equal to their original form, however they are often easier to work with when the denominators are the same.

## Practice

Problem 1
• Current
You have two fractions, start fraction, 2, divided by, 5, end fraction and start fraction, 3, divided by, 10, end fraction, and you want to rewrite them so that they have the same denominator (and whole number numerators).
What number(s) could you use for the denominator?

Want to try more problems like this? Check out this exercise.

## Want to join the conversation?

• I get it. If a anyone is do this right now, click the upvote.
• If I don't get it than do I click downvot?
• At first I was really confused with the least common denominator Q's. Then i realised that I had to find the number which was in both multiples. Some questions can be answered like this:

Oh, 3 times 5 is 15! yas

But can you do it another way? (not for the one which you have to times the denominators, but like, the other types of questions)
?/6 and ?/4, something like that. Hope you understand me XD :3
• Well, yes I did have the same question, but sorry, that’s the only way!
(You can convert the fractions into decimals/percentage then multiply, but that might overcomplicate the equation!)
Have a great day!
• How do you find a common denomenator for 2 fractions like 1/5 and 2/6?
• You would just keep listing all the multiples until you find a common one, so both 5 and 6 are multiples of 30, so the common denominator would be 30
• That was sorta hard but fun
• Im glad you enjoyed it! :)
(1 vote)
• i will give robux to enyone up votes this right now!
• give me 10k bobux rn or i down vote
• If you don't understand just ask the teacher! 😘
• Who you in love with? 😘 That the emoji you did. Wait... do you like ur teacher? Nahhhh BRO!
• Are two fractions multiplied equals 1 called reciprocals?