What is the change in rate from 1950 to 2000

Ineed with number 17 i don’t get it at alll

Icould really use some with this question.

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?