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JEE Main & Advanced JEE Main Paper (Held On 11-Jan-2019 Evening)

  • question_answer
    The number of functions f from {1, 2, 3,...., 20} onto {1, 2, 3,..., 20} such that f(k) is a multiple of 3, whenever k is a multiple of 4 is [JEE  Main Online Paper (Held on 11-jan-2019 Evening)]

    A) \[{{6}^{5}}\times (15)!\]                                  

    B) \[(15)!\times 6!\]

    C) \[{{5}^{6}}\times 15\]             

    D)   \[5!\times 6!\]

    Correct Answer: B

    Solution :

    Here, f(k)=multiple of 3(3,6,9,12,15,18) For k = 4, 8, 12, 16, 20 there are \[6\times 5\times 4\times 3\times 2=6!\]ways For rest numbers, there are 15! ways \[\therefore \]Total ways \[=6!(15!)\]


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