Permutation (order matters): How many ways to arrange r items from n? [formula] Combination (order doesn't matter): How many ways to choose r items from n…
Permutation (order matters): How many ways to arrange r items from n? [formula] Combination (order doesn't matter): How many ways to choose r items from n? [formula] Quick rule: "Order matters → Permutation. Order doesn't → Combination." Example: Picking a president + VP from 10 people → P(10,2) = 90 (order matters). Picking a committee of 2 from 10 → C(10,2) = 45 (order doesn't matter).
Example: How many ways can 3 books be arranged on a shelf from 8 books?
The ACT tests permutations/combinations but the SAT doesn't. Key decision: ask 'Does the order of selection matter?' If picking roles (president, VP) → permutation. If picking a group (committee) → combination. When in doubt, think: 'Would rearranging the selected items create a different outcome?'
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