Consider two boxes: A and B. Box A contains 8 Red and 2 Blue marbles. Box B contains 2 Red and 8 Blue Red marbles. Jack tosses a fair die. If the die comes up with the numbers 1 or 2, then he will choose Box A, if it comes up with 3, 4, 5 or 6, then he chooses Box B. Once a box has been chosen, he will reach inside and randomly pick one of the ten marbles in that box. Suppose, Jack has picked a red marble. What is the probability that the red marble came from Box B?
A) 1/3.
B) 1/4.
C) 1/2.
D) 1/5.

Question

Quizplus · Accepted Answer

Jack has a 1/2 chance of choosing Box A and a 1/2 chance of choosing Box B. If he chooses Box A, the probability of picking a red marble is 8/10. If he chooses Box B, the probability of picking a red marble is 2/10. Therefore, the probability that the red marble came from Box B is (1/2) * (2/10) = 1/10.

Consider Two Boxes: a and B