Question 470498
N(A) = N((A and B') U (A and B)) = N(A and B') + N(A and B) = 15 + 2 = 17

N(B) = N((A' and B) U (A and B)) = N(A' and B) + N(A and B) = 50 + 2 = 52

==> N( A U B) = N(A) + N(B) - N(A and B) = 17 + 52 - 2 = 67

==> N((A U B)') = N(S) - N(A U B) = 120 - 67 = 53.