To solve this problem we change the structure from
1/2 + ? = 7/8
to
7/8 - 1/2 = ?
Now we need to subtract the fractions and find the difference. To do this, make the denominators (the bottom numbers on the fractions) the same.
1/2 = 4/8
then subtracts the numerator
7/8 - 4/8 = 3/8
there is your answer... 3/8