JEE Main & Advanced Mathematics Question Bank Critical Thinking

  • question_answer
    Urn A contains 6 red and 4 black balls and urn B contains 4 red and 6 black balls. One ball is drawn at random from urn A and placed in urn B. Then one ball is drawn at random from urn B and placed in urn A. If one ball is now drawn at random from urn A, the probability that it is found to be red, is                                                   [IIT 1988]

    A)                 \[\frac{32}{55}\]  

    B)                 \[\frac{21}{55}\]

    C)                 \[\frac{19}{55}\]  

    D)                 None of these

    Correct Answer: A

    Solution :

               Let the events are            \[{{R}_{1}}=A\] red ball is drawn from urn \[A\] and placed in \[B\]            \[{{B}_{1}}=A\] black ball is drawn from urn A and placed in B            \[{{R}_{2}}=A\] red ball is drawn from urn \[B\] and placed in \[A\]            \[{{B}_{2}}=A\] black ball is drawn from urn B and placed in A            \[R=A\] red ball is drawn in the second attempt from \[A\]            Then the required probability            \[=P({{R}_{1}}{{R}_{2}}R)+({{R}_{1}}{{B}_{2}}R)+P({{B}_{1}}{{R}_{2}}R)+P({{B}_{1}}{{B}_{2}}R)\]            \[=P({{R}_{1}})P({{R}_{2}})P(R)+P({{R}_{1}})P({{B}_{2}})P(R)+P({{B}_{1}})P({{R}_{2}})P(R)+\]            \[P({{B}_{1}})P({{B}_{2}})P(R)\]            \[=\frac{6}{10}\times \frac{5}{11}\times \frac{6}{10}+\frac{6}{10}\times \frac{6}{11}\times \frac{5}{10}+\frac{4}{10}\times \frac{4}{11}\times \frac{7}{10}+\frac{4}{10}\times \frac{7}{11}\times \frac{6}{10}\]                 \[=\frac{32}{55}\].


You need to login to perform this action.
You will be redirected in 3 sec spinner