Box 1 contains 4 white & 2 black balls and box 2 contains 1 white & 3 black balls. A ball is transferred from box 1 to box 2. A ball is withdrawn from box 2 and it is found to be black. What is the probability that transferred ball from box 1 was white? I don't understand whether we have to find the probability of white ball from box 2 or from box 1?