Given, U is the universal set with 100 elements; A and B are two sets such that n(A) = 50, n(B) = 60, n(A ∩ B) = 20 We know that, n(A' ∩ B') = n(A') + n(B') - n (A' B') .... (1) Now, n(A') = n(U) - n(A) = 100 - 50 = 50 n(B') = n(U) - n(B) = 100 - 60 = 40 n (A' B') = n(U) - n(A ∩ B) = 100 - 20 = 80 Equation (1) becomes, ⇒n(A' ∩ B') = 50 + 40 - 80 ⇒n(A' ∩ B') = 10 Hence, if U is the universal set with 100 elements; A and B are two sets such that n(A) = 50, n(B) = 60, n(A ∩ B) = 20 then n(A' ∩ B') = 10