1. The problem statement, all variables and given/known data A Bag contains 4 balls.two balls are drawn at random,and are found to be white.what is the probability that all balls are white? 2. Relevant equations P(B/A) = P(A intersection B) / P (A) 3. The attempt at a solution A = Two balls are white. B = All four balls are white. I wrote P(B/A) and P(A/B) according to Bayes' theorem and eliminated the intersection term, to arrive with P(B/A) = P(B)/P(A). Ofcourse, P(A|B) = 1. How do i solve these two now? I found an alternate solution on net which goes this way... and another which says... Can anyone, apart from helping me with my question, explain the first aliter and the mistake in assuming the probability = 1/2 in the second aliter?