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Common denominators can be very useful when you are doing addition and subtraction with fractions. So what are common denominators and how do you find them? Common denominators are two denominators of two fractions that are common. So let's take the two fractions 3/4 and 3/5 and to find the common denominators for the two fractions we will have to write the fractions out in vertical form like this:

3
4

5
6

So now that we have the two fractions we have to take the denominators and find the LCM of the two numbers like this:
3
4                            4, 8, 12, 16, 20
=  4 and 6 =                                  LCM=12
5                            6, 12, 18, 24, 30
6

We know that the LCM of the two denominators is 12 so now we have to rename the fractions like this:
3     = ?
4        12

5     = ?
6        12

So now we have to rename the numerators by multiplying them by the same we multiplied each denominator by to get 12 like this:

3
4         12

=4 x 3=12

5         10
6         12

=6 x 2=12

We now know that the common denominators for the two fractions 3/4 and 5/6 is 12. And so our fractions with common denominators are 9/12 and 10/12.
Now try some of these and make sure to watch the video on YouTube.

Sheet 1 How to find Common Denominators

Sheet 2 How to find Common Denominators