1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
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: Counting Permutations

  1. Sep 2, 2007 #1
    1. The problem statement, all variables and given/known data
    How many 3 digit numbers can be constructed from digits 1, 2, 3, 4, 5, 6, and 7 if each digit may be used once only and the number is odd?

    2. The attempt at a solution
    What number do they speak of? The resulting 3 digit number? How do I approach this equation?
  2. jcsd
  3. Sep 2, 2007 #2


    User Avatar
    Homework Helper

    For example, I could pick out the numbers 1,2 and 3 and form the number


    but also, I could form, 132 ,or 213, or 231, or 312, or 321.

    But, that is just one way to pick three numbers (1,2,3). I could have chosen to pick out the numbers 3,5 and 1. And I could then form 6 different numbers (135,153,315,351,513,531) with those.

    If I were you I would start off by thinking about how many different ways there are to choose three different things out of an array of 7 different things. "Seven choose three".

    Then, you know that for any set of three, you can make 6 numbers, but you have to figure out how many of them are odd. Good luck.
  4. Sep 2, 2007 #3


    User Avatar
    Homework Helper

    Oh. Yeah. They are probably talking about the *resulting* number (the three digit number). That is a very confusing way to word the problem. It is certainly vague.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook