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Thread: How many permutations of [9] have no adjacent odd digits?

  1. #1
    Member oldguynewstudent's Avatar
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    How many permutations of [9] have no adjacent odd digits?

    How many permutations of [9] have no adjacent odd digits?

    First look at the same problem for [5] we have:
    12345 12543 14325 14523
    32145 32541 34125 34521
    52341 52143 54321 54123

    This suggests that for each odd digit we have a permutation of the rest of the even digits times the permutation of the rest of the odd digits (I think) giving

     <br />
3*\left(2\right)_{2}*\left(2\right)_{2}<br />
= 12

    Then the answer to the original question would be

     <br />
9*\left(4\right)_{4}*\left(4\right)_{4}<br />
= 5184

    Does this solution hold any merit or am I totally confused as usual?

    Thanks
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  2. #2
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    Notice that in [9] there are five odd numbers and four even.
    The even 'separate' the odds: "_2_4_6_8_". Fill in the blanks with the odds.
    There (4!) ways to arrange the evens and then (5!) ways to arrange the odds.
    SO?
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