If A and B are 2 sets such that A U B has 40 elements, A has 18 elements and B has 29 elements, how many elements does A ∩ B have?
Since n(A U B) = n(A) + n(B) - n(A ∩ B)
Given : n(A U B) =40, n(A) =18, n(B) =29
Hence, n(A ∩ B) = n(A) + n(B) - n(A U B)
= 18 + 29 - 40
= 7