Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    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


    User Avatar
    Science Advisor
    Homework Helper

    Seems fine to me.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook