How do I solve these problems?

Pls! does anybody know a shortcut for answering these types of questions in the future?

The sum of two numbers is 12 and their difference is 18 what are the two numbers

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?