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Math Help - Counting principle

  1. #1
    Member
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    Sep 2009
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    Counting principle

    Hi,

    How many integers are there between 100 and 1000 all of whose digits are odd?

    I know the answer to this: 125.

    How is the counting principle to be used in solving this?
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  2. #2
    Senior Member
    Joined
    Dec 2010
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    There are 5 odd digits: 1, 3, 5, 7, 9
    How many numbers with all odd digits are between 100 and 1000?
    Isn't this just how many 3 digit numbers have all odd digits?

    # 3 odd digit numbers = (# possible odd digits for 1st digit) x (# possible odd digits for 2nd digit) x (# possible odd digits for 3rd digit)
    = 5 x 5 x 5 = ...
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