451.Letters to the Wrong Firms
★★★expected valueCh. 6 — Random Variables and Expectation444+ Problems in Probability
You wrote letters for recruiters at different firms, then forgot which recruiter belongs to which firm and assigned the letters to firms uniformly at random, one letter per firm. What is the expected number of letters that reach the correct firm?
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Indicator variables — no need to touch derangement counts.
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