$U_1$, $U_2$, $U_3$ are three urns. $U_1$ contains 5 red, 3 white, 2 black balls; $U_2$ contains 4 red, 4 white, 2 black balls; and $U_3$ 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