College Algebra / Unit 9 Alg 515 short lesson, about 20 seconds
Permutations
Captions available
The idea
A permutation counts ordered selections: choosing and arranging r objects from n distinct ones gives P(n, r) = n!/(n − r)! ways.
In practice
P(5, 2) = 5!/3! = 20 — five choices, then four, for two ranked seats.
Where people slip
Order matters here; a swapped pair counts twice.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
Builds on
- Nothing. This is a starting point.
Unlocks
- Nothing yet depends on this.
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