Question

Twenty add something equals 100 divided by four

Answer 1

20 + x = 100/4
20 + x = 25
x = 25 - 20
x = 5

or you could put:
20 + 5 = 100/4

Answer 2

Five. Just divide 100 by four, to get 25, then subtract 20 from 25 to get your answer