Select the question that is different from the other three. And also find both answers

Match each expression with its simplified form.

Sum is 17 and their difference is 3

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?