3/8 of a box of balls are red, 3/10 of them are green and the rest are black. If there are 36 green balls, how many black balls are there?
n = total balls
36 = 0.30 n
n = 120 total
red = 3/8 * 120 = 45
so green + red = 36+45 = 81
120 - 81 = 39 = black