框P有2个红球和3个蓝球，框Q有3个红球和1个蓝球。选择一个球如下：
(i) Select a box
(ii) Choose a ball from the selected box such that each ball in
the box is equally likely to be chosen. The probabilities of
selecting boxes P and Q are (1/3) and (2/3), respectively. 假定在上述过程中选择的球是红色球，则它来自框P的概率为
(A) 4/19
(B) 5/19
(C) 2/9
(D) 19/30答案： (A)
解释：
The probability of selecting a red ball =
(1/3) * (2/5) + (2/3) * (3/4) = 2/15 + 1/2
= 19/30 [Let it be P1]
Probability of selecting a red ball from box P =
(1/3) * (2/5) = 2/15 [Let it be P2]
Given that a ball selected in the above process is
a red ball, the probability that it came from the
box P is = P1/P2 = (2/15) / (19/30) = 4/19 这个问题的测验