The Careless Mailing Clerk

After 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?

Solution

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Copyright © 1996-2017 Alexander Bogomolny

If nine letters are in the correct envelopes, the tenth must be also, so the probability is zero.

References

  1. C. W. Trigg, Mathematical Quickies, Dover, 1985, #1

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Copyright © 1996-2017 Alexander Bogomolny

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