Results 1 to 3 of 3

Math Help - Maddening Unexplained Conditional Permutations

  1. #1
    Junior Member
    Joined
    Nov 2008
    Posts
    62

    Maddening Unexplained Conditional Permutations

    Hey, the textbook decided to throw out some problems that it hadn't explained in the lesson on conditional permutations. I've been trying to figure it out myself, but i could really use some help.

    Alright, it says "How many positive integers are there less than 900 such that all digits are odd?"

    It explained how to do like three-digit numbers, but not individual integers.. Help would be totally appreciated!
    Follow Math Help Forum on Facebook and Google+

  2. #2
    MHF Contributor

    Joined
    Aug 2006
    Posts
    18,792
    Thanks
    1687
    Awards
    1
    There are five odd digits: {1,3,5,7,9}.
    But 9 cannot be the first digit. So there are (4)(5)(5)=100.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Junior Member
    Joined
    Nov 2008
    Posts
    62
    Thanks Plato, but there's where i got my hangup. that solves for only three-digit numbers. not all integers.

    actually, i think i just figured it out.

    only using odd numbers so 5 is the greatest number of variables.

    under 900 so 4 possible numbers the first digit.

    4 x 5 x 5 = 100 (first possible outcome) then add another row for the two digit possible outcomes.
    5 x 5 = 25 repeat for single digit...
    5 = 5

    100+25+5 = 130. 130 odd integers lower than 900.

    eh?
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Replies: 0
    Last Post: July 22nd 2011, 01:39 AM
  2. permutations
    Posted in the Discrete Math Forum
    Replies: 4
    Last Post: January 23rd 2010, 05:13 AM
  3. Permutations again
    Posted in the Statistics Forum
    Replies: 7
    Last Post: July 12th 2008, 02:39 PM
  4. Permutations
    Posted in the Statistics Forum
    Replies: 1
    Last Post: April 13th 2008, 07:23 AM
  5. Replies: 2
    Last Post: November 10th 2007, 12:55 AM

Search Tags


/mathhelpforum @mathhelpforum