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Math Help - no letter in right mailbox

  1. #1
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    no letter in right mailbox

    Help me this problem.
    A postman has 9 letters. If he puts randomly to 9 mailboxes, then calculate probability that there is no letter in right mailbox.
    Thanks a lot.
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  2. #2
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    Quote Originally Posted by mahefo View Post
    Help me this problem.
    A postman has 9 letters. If he puts randomly to 9 mailboxes, then calculate probability that there is no letter in right mailbox.
    Thanks a lot.
    This simply a problem in derangements .
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  3. #3
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    Hint:
    1 2 3 4 5 6 7 8 9
    1 2 3 4 5 6 7 9 8
    ...
    9 8 7 6 5 4 3 1 2
    9 8 7 6 5 4 3 2 1
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