JEE Main & Advanced JEE Main Paper (Held On 11 April 2015)

  • question_answer
    Let \[A=\{{{x}_{1}},{{x}_{2}},...,{{x}_{7}}\}\]and \[B=\{{{y}_{1}},{{y}_{2}},{{y}_{3}}\}\]be two sets containing seven and three distinct elements respectively. Then the total number of functions\[f:A\to B\] that are onto, if there exist exactly three elements x in A such that\[f(x)={{y}_{2}},\] is equal to : [JEE Main Online Paper (Held On 11 April 2015)]  

    A) \[14.{{\,}^{7}}{{C}_{2}}\]

    B) \[16.{{\,}^{7}}{{C}_{3}}\]

    C) \[12.{{\,}^{7}}{{C}_{2}}\]

    D) \[14.{{\,}^{7}}{{C}_{3}}\]

    Correct Answer: D

    Solution :

      Selection of three element in A such that \[f\left( x \right)={{y}_{2}}{{=}^{7}}{{C}_{3}}\] Now for remaining 4 elements in A we have 2 elements in B \[\therefore \]Total number of onto function \[{{=}^{7}}{{C}_{3}}\times \left( {{2}^{4}}-2{{C}_{1}}{{\left( 2-1 \right)}^{4}} \right){{=}^{7}}{{C}_{3}}\times 14\]


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