Three urns f/i, U2, U3 contain, respectively, two white and three black
balls, three white and four black balls, and one white and two black
balls.
(a) Two balls, one from urn U\ and the other from urn t/2, are drawn
and placed in urn U3; thereafter one ball is drawn from U3 and is
found to be black. What is the probability that the ball drawn
from Ui was white?
(b) One ball is drawn from Ux and placed in U2\ thereafter two balls
are drawn from U2 and placed in U3. Then a ball is drawn from
U3 and is found to be white. What is the probability that the two
balls drawn from U2 are both white?