The ball park is selling a hamburger combo. At the end of the day, they collected $103.95. If a total of 21 hamburger combos were sold, how much did each combo cost?

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?