Let A and B be subsets of a universal set U and suppose n(U) = 390, n(A) = 130, n(B) = 90, and n(A B) = 40. Find the number of elements in the set.

Question

Let A and B be subsets of a universal set U and suppose n(U) = 390, n(A) = 130, n(B) = 90, and n(A  B) = 40. Find the number of elements in the set. 
n(Ac n Bc)

oobleck · Accepted Answer

390-(130+90-40) = ____ Ac ∩ Bc = (A U B)c in other words, everything in U which is not in A or B or both.