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Math Help - Solve for n, permutation problem

  1. #1
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    Unhappy Solve for n, permutation problem

    Solve for n.
    P(n,5)=20P(n,3)

    I'm leaning this course by myself, and i'm finding it difficult to understand how to do this problem with the material that was given to me in my book. Any help would be greatly appreciated because so far all i'm understanding is that i plug the numbers in to n!/(n-r)!
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  2. #2
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    Quote Originally Posted by youvy View Post
    Solve for n.
    P(n,5)=20P(n,3)

    I'm leaning this course by myself, and i'm finding it difficult to understand how to do this problem with the material that was given to me in my book. Any help would be greatly appreciated because so far all i'm understanding is that i plug the numbers in to n!/(n-r)!
    n!/(n-5)! = 20n!/(n-3)!

    1/(n-5)! = 20/(n-3)!

    1 = 20/ (n-4)(n-3)

    n^2 - 7n + 12 = 20

    n^2 - 8n + n - 8 =0

    (n -8)(n+1) =0

    Thus answer is 8
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