Q5. 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?
n( X Y) = 60
n( X ∩ Y) = 10
n(X) = 40
n( X Y) = n(X) + n(Y) - n( X ∩ Y)
n(Y) = 60 - 40 + 10
n(Y) = 30
Hence,set Y has 30 elements.