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Chatty reference · stats1 week 6

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2026-08-16

26. Stats W6 — Permutations & Combinations

Permutation:

P(n,r)=\frac{n!}{(n-r)!}

Combination:

=\frac{n!}{r!(n-r)!}$$ ### Recognition rule Ask: > **Does order matter?** If yes → permutation-type reasoning. If no → combination-type reasoning. This becomes a major pattern classifier. ---