The distance formula, , is the key element in the proof of which of the following? a. cpctc b. the law of cosines c. the law of sines d. the triangle inequality theorem

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?