Expectation · Mail · difficulty 2 of 5
Letters in the right envelopes
You have 10 letters and 10 envelopes, each envelope addressed to match exactly one letter. The letters are put into the envelopes completely at random, one letter per envelope, with every arrangement equally likely. What is the expected number of letters that end up in their own envelope?
Answer with an expected value. First lock in a gut guess with an 80% range, then solve it properly, then watch a simulation settle on the truth.
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