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

  • question_answer
    A mapping is selected at random from the set of all the mappings of the set \[A=\left\{ 1,\,\,2,\,...,\,n \right\}\]into itself. The probability that the mapping selected is an injection is

    A)   \[\frac{1}{{{n}^{n}}}\]                 

    B)   \[\frac{1}{n\,!}\]

    C)   \[\frac{(n-1)\,!}{{{n}^{n-1}}}\]    

    D)            \[\frac{n\,!}{{{n}^{n-1}}}\]

    Correct Answer: C

    Solution :

                       The total number of functions from \[A\] to itself is \[{{n}^{n}}\] and the total number of bijections from \[A\]to itself is \[n\,\,!.\] {Since \[A\] is a finite set, therefore every injective map from \[A\] to itself is bijective also}.                    \[\therefore \]The required probability \[=\frac{n\,\,!}{{{n}^{n}}}=\frac{(n-1)\,\,!}{{{n}^{n-1}}}.\]


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