Question
There are three urns A, B and C. Urn A contains 4 white balls and 5 blue balls. Urn B contains 3 white balls and 4 blue balls. Urn C contains 3 white balls and 6 blue balls. One ball is drawn from each of these urns. What is the probability that out of these three balls drawn, two are white balls and one is a blue ball?
Solution
Urn A Urn B Urn C
White 4 3 3
Blue 5 4 6
P(two third and one blue) = P(WWB) + P(WBW) + P(BWW)
= P(W) P(W) P(B) + P(W) P(B) P(W) + P(B) P(W) P(W) 
