Let A = {a1,a2,...,an} For a1 ∈ A, we have the following choices: (i) ai ∈ P; ai ∈ Q (ii) ai ∈ P; ai ∈ Q (iii) ai ∈ P; ai ∈ Q (iv) ai ∈ P; ai ∈ Q Out of these only (ii), (iii) and (iv) imply ai ∉ P ∩ Q. Hence, the number of ways in which none of a1,a2,...,an belong to P ∩ Q is 3n.