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Math Help - Permutation question!

  1. #1
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    Permutation question!

    Hi guys, The question is 7Pn=42 from here i changed to \7!(7-n)!=42 and crossed multiplied giving me the wrong answer the right answer however is two how do we do these?
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  2. #2
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    Re: Permutation question!

    Quote Originally Posted by Gurp925 View Post
    Hi guys, The question is 7Pn=42 from here i changed to \7!(7-n)!=42 and crossed multiplied giving me the wrong answer the right answer however is two how do we do these?
    It's actually
     _7 P _n = \frac{7!}{(7 - n)!} = 42 \implies 7! = 42 \cdot (7 - n)!

    Or you could reduce
    \frac{7!}{(7 - n)!}
    directly.

    -Dan
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  3. #3
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    Re: Permutation question!

    Quote Originally Posted by Gurp925 View Post
    Hi guys, The question is 7Pn=42 from here i changed to \7!(7-n)!=42 and crossed multiplied giving me the wrong answer the right answer however is two how do we do these?
    You might just do it by inspection: 7\cdot 6=42. So what is n=~?
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    Re: Permutation question!

    love Plato's method answer = 2 yay! and once we get to this portion 7!=42(7-n)! how do we get the answer two from here ?
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    Re: Permutation question!

    Quote Originally Posted by Gurp925 View Post
    love Plato's method answer = 2 yay! and once we get to this portion 7!=42(7-n)! how do we get the answer two from here ?
     \begin{align*} 7! &= 42(7-n)!\\6! &=6(7-n)!\\ 5! &=(7-n)!\end{align*}

    So n=~?
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  6. #6
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    Re: Permutation question!

    Plato has explained very well. However we can also proceed as under
    7! = 42 * ( 7-n)!
    7*6* 5! = 42 * ( 7-n)!
    5! = ( 7-n)!
    that gives 7-n=5 OR n = 2
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  7. #7
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    Re: Permutation question!

    Great answers thanks so much guys.
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