To find a common denominator, multiply each part of a fraction by the denominator of the other fraction. For example, to find a common denominator for 1/3 + 2/5, multiply the 1 and 3 by 5 and multiply the 2 and 5 by 3. You should get 5/15 + 6/15. Then you can calculate the fractions.
I'm not sure if this is what you meant but here