A manufacturer has two factories which make watches, A and B. Defective watches are made 1 out of 100 times in A, and 1 out of 200 times in B. A retailer gets a large box of watches shipped from one of such factories, but he doesn't know which one. Assume he tries a watch from box and it works. Determine the probability that the second watch he tries from the box works, given the first did?