Elsa has a bag of sweets containing 7 chocolates, 8 toffees and 3 liquorice laces. She takes out 2 sweets at random and eats them. They are both chocolate.
What fraction of the sweets left in the bag are toffees?

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?