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JEE Advanced 2017 Paper 2 · Mathematics · Probability · medium

Urn A contains 3 red and 2 black balls; urn B contains 2 red and 4 black balls. if the number is 1 or 2 urn A is chosen, otherwise urn B is chosen.…

Single correct option

Urn AA contains 33 red and 22 black balls; urn BB contains 22 red and 44 black balls. A fair die is thrown: if the number is 11 or 22 urn AA is chosen, otherwise urn BB 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 AA is
  1. A. 911\dfrac{9}{11}
  2. B. 913\dfrac{9}{13}
  3. C. 34\dfrac{3}{4}
  4. D. 59\dfrac{5}{9}
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Answer: (B) 913\dfrac{9}{13}

Use Bayes' theorem with unequal priors: P(A)=26=13P(A)=\tfrac{2}{6}=\tfrac13 and P(B)=23P(B)=\tfrac23. For urn AA, P(RR∣A)=35⋅24=310P(RR\mid A)=\tfrac{3}{5}\cdot\tfrac{2}{4}=\tfrac{3}{10}. For urn BB, P(RR∣B)=26⋅15=115P(RR\mid B)=\tfrac{2}{6}\cdot\tfrac{1}{5}=\tfrac{1}{15}. Then P(A∣RR)=13⋅31013⋅310+23⋅115=1/101/10+2/45=9/909/90+4/90=913P(A\mid RR)=\dfrac{\frac13\cdot\frac{3}{10}}{\frac13\cdot\frac{3}{10}+\frac23\cdot\frac1{15}}=\dfrac{1/10}{1/10+2/45}=\dfrac{9/90}{9/90+4/90}=\dfrac{9}{13}. The option 9/119/11 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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