The set (A' ∩ B) is the set of those elements of B, that do not belong to A. So (A' ∩ B) is simply the difference of two subsets B and (A ∩ B) : (A' ∩ B) = B \ (A ∩ B), (*) where the symbol " \ " symbolizes subtraction of subsets. From (*), n(A' ∩ B) = n(B) - n(A ∩ B) = 60 - 10 = 50. ANSWER