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Probability

Four hats returned with at least one correct

countingderangementsinclusion-exclusion

Four people hang up their hats, and the hats are returned at random, one to each person.

In how many ways does at least one person get their own hat back?

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Balls and Bins and the Birthday ProblemBinomial Coefficients and Pascal's TriangleBurnside's Lemma and Polya CountingCatalan NumbersCounting: Permutations and Combinations

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