Finite math problem, please help me understand

Question

Suppose the universe U has two subsets C and D. n(C') = 90, n(C) = 38, n(D') = 52.

How many elements are in D? n(D) =

Answer 1

n(C) + n(C') = n(U) = 90 + 38 = 128

n(D) = n(U) - n(D') = 128 - 52 = 76