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

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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