If X and Y are two sets such that X has 40 elements, X∪Y has 60 elements and X∩Y has 10 elements, how many elements does Y have?

Given n(X) = 40, n(X∪Y) = 60, n(X∩Y) = 10
We know that:
n(X∪Y) = n(X) + n(Y) - n(X∩Y)
∴ 60 = 40 + n(Y) - 10
∴ n(Y) = 60 - (40 - 10) = 30
Thus, the set Y has 30 elements.