Hello Naresh,

Given, P(B) = 2/7, P(A U B') = 4/5

As A and B are independent events, P(A U B) = P(A) + P(B)

=>P(A U B') = P(A) + 1 - P(B)     [As P(B') = 1 - P(B)]

=>4/5 = P(A) + 1 - 2/7               [P(B') = 1-2/7=5/7]

=>P(A) = 3/35

This is a simple question on probability. Do pay attention to the reasons in the brackets.