JEE Main & Advanced Mathematics Probability Question Bank Self Evaluation Test - Probability-II

  • question_answer
    3 friends A, B and C play the game ?Pahle hum pahle tum? in which they throw a die one after the other and the one who will get a composite number 1st will be announced as winner, if A started the game followed by B and then C then what is the ratio of their winning probabilities?

    A) \[9:6:4\]

    B) \[8:6:5\]

    C) \[10:5:4\]

    D) None of these

    Correct Answer: A

    Solution :

    [a] Probability of getting a composite number is \[2/6=1/3\]
    Probability that A will win the game is
    \[\left( \frac{2}{3} \right)\left( \frac{1}{3} \right)+\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{1}{3} \right)\]\[+\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{1}{3} \right)+....\]
    \[=\frac{\frac{1}{3}}{1-\frac{8}{27}}=\left( \frac{1}{3}\times \frac{27}{19} \right)=\frac{9}{19}\]
    Probability that B will win the game is
    \[\left( \frac{2}{3} \right)\left( \frac{1}{3} \right)+\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{1}{3} \right)\]
    \[+\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{1}{3} \right)+...\]
    \[=\frac{\frac{2}{9}}{1-\frac{8}{27}}=\left( \frac{2}{9}\times \frac{27}{19} \right)=\frac{6}{19}\]
    Probability that C will win the game is
    \[\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{1}{3} \right)+\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{1}{3} \right)\]
    \[+\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{2}{3} \right)\left( \frac{1}{3} \right)+...\]
    \[=\frac{\frac{4}{27}}{1-\frac{8}{27}}=\left( \frac{4}{27}\times \frac{27}{19} \right)=\frac{4}{19}\]
    So required ratio is \[9:6:4,\]


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