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Math Help - [SOLVED] combinatorics

  1. #1
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    [SOLVED] combinatorics

    1. How many 3-digit positive integers are odd and do not contain the digit 5?
    2.
    Martha invited 4 friends to go with her to the movies. There are 120 different ways in which they can sit together in a row. In how many of those ways is Martha sitting in the middle?
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  2. #2
    Member TheMasterMind's Avatar
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    Quote Originally Posted by william View Post
    1. How many 3-digit positive integers are odd and do not contain the digit 5?
    2.
    Martha invited 4 friends to go with her to the movies. There are 120 different ways in which they can sit together in a row. In how many of those ways is Martha sitting in the middle?
    1.4*9*8=288 ways
    2.24*1=24 ways

    ...Can you explain why?
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