There are three bags. Bag A contains 2 white, 1 black & 3 red balls; bag B contains 3 white, 2 black & 4 red balls; bag C contains 4 white, 3 black & 2 red balls. One bag is chosen randomly and 2 balls are drawn from it. If the balls drawn are 1 black & 1 red, then find the probability that they are drawn from bag B.