A leap year consists of 366 days comprising of 52 weeks and 2 days. There are 7 possibilities for these 2 extra days. (i) Sunday, Monday, (ii) Monday, Tuesday, (iii) Tuesday, Wednesday, (iv) Wednesday, Thursday, (v) Thursday, Friday, (vi) Friday, Saturday and (vii) Saturday, Sunday. Let us consider two events: A : the leap year contains 53 Sundays B: the leap year contains 53 Mondays. Then we have P(A) = 2/7, P(B) = 2/7, P(A ∩ B) = 1/7 Hence, required probability = P(A ∪ B) = P(A) + P(B) - P(A ∩ B) = 2/7 + 2/7 - 1/7 = 3/7