Answer 1

P = {1, 2, 5, 7, 9}, Q = {2, 3, 5, 9, 11}, R = {3, 4, 5, 7, 9} and S = {2, 3, 4, 5, 8}

P ∩ Q = {1, 2, 5, 7, 9} ∩ {2, 3, 5, 9, 11}

= {2, 5, 9}

(P ∩ Q) ∩ S = {2, 5, 9} ∩ {2, 3, 4, 5, 8}

= {2, 5}