100 students were interviewed. 72 ate at A, 52 ate at B. How many ate at A and B?

P(A U B) = P(A) + P(B) - P(A intersection B)
100 = 72 + 52 - P(A intersection B)
P(A intersection B) = 72 + 52 - 100
P(A intersection B) = P(who ate at A and B) = 24