Box 1 contains 1000 transistors, of which 100 are defective, and box 2 contains 2000 transistors, of which 100 are also defective. A box is taken at random and two transistors are drawn from it, at random and without replacement.
(a) Calculate the probability that both transistors are defective.
(b) Given that both transistors are defective, what is the probability that they come from box 1?