## 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?

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Copyright © 1996-2018 Alexander Bogomolny

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

### References

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

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Copyright © 1996-2018 Alexander Bogomolny