In order to add fractions the denominators must be the same.
Suppose you want to add:
3/4 + 1/2
You'll need to find the least common multiple (LCM) of the denominators (4 and 2). The least common multiple is 4, so the least common denominator is 4.
Since the last common denominator is 4, we need to change our fraction 1/2 to 2/4.
So our new problem is:
3/4 + 2/4