In an indirect proof to prove that measure of angle P is greater than the measure of angle Q, what is an assumption you must make about measure of angle P and the measure of angle Q.

There are three bags: a (contains 2 white and 4 red balls), b (8 white, 4 red) and c (1 white 3 red). you select one ball at random from each bag, observe that exactly two are white, but forget which ball came from which bag. what is the probability that you selected a white ball from bag a?