JEE Main & Advanced Mathematics Permutations and Combinations Question Bank Self Evaluation Test - Permutations and Combinatioins

  • question_answer
    If a denotes the number of permutations of x+2 things taken all at a time, b the number of permutations of x things taken 11 at a time and c the number of permutations of x -11 things taken all at a time such that \[a=182\,\,bc,\] then the value of x will be

    A) 12

    B) 10

    C) 8

    D) 6

    Correct Answer: A

    Solution :

    [a] \[^{x+2}{{P}_{x+2}}=a\Rightarrow a=(x+2)!\] \[^{x}{{P}_{11}}=b\Rightarrow b=\frac{x!}{(x-11)!}s\] and \[^{x-11}{{P}_{x-11}}=c\] \[\Rightarrow c=(x-11)!;\,\,\,a=182bc\] \[\therefore (x+2)!=182\frac{x!}{(x-11)!}(x-11)!\] \[\Rightarrow (x+2)(x+1)=182=14\times 13\] \[\therefore x+1=13\]          \[\therefore x=12\]


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