Single correct option
Urn contains red and black balls; urn contains red and black balls. A fair die is thrown: if the number is or urn is chosen, otherwise urn is chosen. Two balls are then drawn from the chosen urn one after the other without replacement. Given that both balls drawn are red, the probability that they came from urn is
- A.
- B.
- C.
- D.
Show answer and solution
Answer: (B)
Use Bayes' theorem with unequal priors: and . For urn , . For urn , . Then . The option results from wrongly using equal priors, and the others come from mishandling the without-replacement probabilities. The key idea is that the prior probabilities are not equal because the die gives urn A only two of six faces, so the likelihoods must be weighted before normalising by the total probability of drawing two red balls.
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