A + B : A is sister of B. A/B : A is son of B. A = B : A is brother of B. A @ B : A is father of B. Now, P @ B = S @ M = N. P is father of B, who is brother of S and S is father of M and M is brother of N. (+) P - (+) B - S (+) = (+) M - N Hence, M is grandson of P.