Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

Two urns, named A and B, contain balls. Urn A contains 10 balls numbered 1-10, and urn B contains 25 balls numbered 1-25. An experiment is performed, in which an urn is chosen at random, and then the ball is chosen from the urn. Suppose that the ball chosen has the number 5. What is the probability that the ball came from urn A?

This is a practice exam, so the exact answer is not as important as the proper solution.

share|cite|improve this question
up vote 2 down vote accepted

Applying Bayes' theorem:

$$P(A|5) = {P(5|A)P(A) \over {P(5|A)P(A) + P(5|B)P(B)}} = {0.1\times0.5 \over {0.1\times0.5 + 0.04\times0.5}} = {5 \over 7}$$

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.