Permutations
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State
A permutation counts ordered selections: choosing and arranging r objects from n distinct ones gives P(n, r) = n!/(n − r)! ways.
Show
P(5, 2) = 5!/3! = 20 — five choices, then four, for two ranked seats.
Watch for
Order matters here; a swapped pair counts twice.
Builds on
- Nothing — this is a starting point.
Unlocks
- Nothing yet depends on this.