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Math Help - Factorials

  1. #1
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    Factorials

    A 12-volume encyclopedia is to be placed on a shelf. How many incorrect arrangements are there?

    This is my work:

    12!=479 001 600

    But their answer is
    12! - 1 =479 001 599

    My question: Why did they subtract 1 from 12! ?

    I know that 12! means how many possible ways you can arrange them correctly, but why subtract 1 for incorrect ways? Could you use any number to subtract away from 12! ? and then that would be considered incorrect?
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  2. #2
    Super Member craig's Avatar
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    Read your question carefully.

    Quote Originally Posted by skeske1234 View Post
    How many incorrect arrangements are there?
    Your actual working out is correct, there are 12! ways to arrange them. However, only one of these is the correct way, hence the subtract one.

    Hope this helps
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  3. #3
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    what do you mean when you say "only one of these" is the correct way?
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  4. #4
    Super Member craig's Avatar
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    Well I presume that the correct arrangement would be volume 1,2,...12. This sequence only occurs once out of all 479,001,600, this would explain why the answer for the number of 'incorrect' ways is is 479,001,599.
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