An urn contains w1 white and b1 black balls. A second urn contains w2 white and b2 black balls. A ball is selected at random from the first urn and placed in the second urn. Then a ball is selected from the second urn.
Find:
1. Probability that is it white.
2. Assume that the ball selected from the second urn is white. Find the probability that it came from the first urn.