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The Careless Mailing ClerkAfter a typist has written ten letters and had addressed the ten corresponding envelopes, a careless mailing clerk inserted the letters in the envelopes at random, one letter per envelope. What is the probability that exactly nine letters were inserted in the proper envelopes? 
Copyright © 1996-2008 Alexander Bogomolny
If nine letters are in the correct envelopes, the tenth must be also, so the probability is zero. References
Copyright © 1996-2008 Alexander Bogomolny
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