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Homework Help: Permutations combinatorics problem?

  1. Jan 19, 2012 #1
    1. The problem statement, all variables and given/known data

    A telephone extension has four digits, how many different extensions are there with no repeated digits, if the first digit cannot be zero?

    2. Relevant equations

    P(n,r)=n!/(n-r)!


    3. The attempt at a solution

    For the first digit, there are 9 possibilities (because no zero)
    For the last 3 digits I used P(9,3)=9!/6!=9x8x7

    So, in the end my result was: 9x9x8x7= 4,536 different extensions...

    I'm just wondering if I was correct?
     
  2. jcsd
  3. Jan 19, 2012 #2

    Dick

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    Science Advisor
    Homework Helper

    Seems fine to me.
     
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