U1,U2,U3 are three urns. U1 contains 5 red, 3 white, 2 back balls; U2 contains 4 red, 4 white, 2 black balls and U3 contains 3 red, 4 white, 3 black balls. If a ball is chosen at random from an urn chosen at random, then the probability of not getting a black ball is