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2006 SMT/General Problems/Problem 3

Revision as of 20:45, 15 October 2013 by DANCH (talk | contribs) (Created page with "==Problem== 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, on...")
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Problem

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

For a letter to be in the wrong envelope, another letter must also be out of its correct envelope, so it is not possible to have exactly nine in the correct envelopes. Therefore, the answer is $\boxed{0}$.

See Also

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