Simplify 15 to the 18th power over 15 to the 3rd power.
15^6
15^15
1^6
1^15

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?