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

  • question_answer
    There is a five-volume dictionary among 50 books arranged on a shelf in random order. If the volumes are not  necessarily kept side by side, the probability that they occur in increasing order from left to right is:

    A) \[\frac{1}{5}\]

    B) \[\frac{1}{{{5}^{50}}}\]

    C) \[\frac{1}{{{50}^{5}}}\]

    D) None of these  

    Correct Answer: D

    Solution :

    [d] The number of ways of arranging 50 books \[{{=}^{50}}{{P}_{50}}=50!\]. The number of ways of choosing places for the vive volume dictionary is \[^{50}{{C}_{5}}\] and the number of ways of arranging the remaining 45 books \[{{=}^{45}}{{P}_{45}}=(45)!\] Thus the number of favourable ways is \[{{(}^{50}}{{C}_{5}})(45!).\] Hence the probability of he required event \[=\frac{\left( ^{50}{{C}_{5}} \right)(45!)}{50!}=\left( \frac{50!}{5!45!} \right)\left( \frac{45!}{50!} \right)=\frac{1}{5!}=\frac{1}{120}\]


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