JEE Main & Advanced Mathematics Probability Question Bank Use of permutations and combinations in probability

  • question_answer
    The letter of the word `ASSASSIN' are written down at random in a row. The probability that no two S occur together is    [BIT Ranchi 1990; IIT 1983]

    A)            \[\frac{1}{35}\]                       

    B)            \[\frac{1}{14}\]

    C)            \[\frac{1}{15}\]                       

    D)            None of these

    Correct Answer: B

    Solution :

                       Total ways of arrangements \[=\frac{8\,!}{2\,!\,\,.\,\,4\,!}\]                    \[\bullet \,w\bullet x\bullet y\bullet z\bullet \]                    Now ?S? can have places at dot?s and in places of \[w,\,\,x,\,\,y,\,\,z\] we have to put \[2\,A's,\] one \[I\] and one \[N.\]                    Therefore favourable ways \[=5\text{ }\left( \frac{4\,!}{2\,!} \right)\]                    Hence required probability \[=\frac{5\,.\,4\,!\,\,2\,!\,\,4\,!}{2\,!\,\,8\,!}=\frac{1}{14}.\]


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