How to Find Common Denominators

Introduction

Finding common denominators is the main thing when adding and/or subtracting fractions. Therefore, this topic is very important, too! 

The key to finding a common denominator is common multiples. All you have to do is find a common multiple of both denominators and you should be on you way!

Find Common Denominators!

Let's say that we have 6/9-3/5.

Remember, all you do is find a common multiple of the denominators.

9= 9,18,27,36,45

5= 5,10,15,20,25,30,35,40,45

As you can see, the LCM is 45. Therefore our new denominators are 45. Now all we do is multiply the numerator by the number we multiplied the denominator by to get 45:

6/9= 30/45 because you multiply by 5 to get 45.

3/5= 27/45 because you multiply by 9 to get 45.

Therefore, we now have 30/45-27/45.

The last step is simple: subtract the numerators. (Remember that the denominators stay the same.)

30-27=3.

The difference of 6/9-3/5= 3/45

 Sheet 1 Practice Problems on Finding Common Denominators