Common Denominator

This is when two or more fractions have the same denominator (the number on the bottom).

If the denominators are not the same (not "common") you cannot add or subtract the fractions.

You can find a common denominator by trying different multiples of the fractions.

Example: you can't add 1/3 and 1/2, but if you multiply 1/3 by 2/2 and multiply 1/2 by 3/3 you get 2/6 and 3/6. They now have a common denominator and you can add them (answer: 5/6)