College Algebra / Unit 9 Alg 516 short lesson, about 20 seconds
Combinations
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The idea
A combination counts unordered selections: choosing r objects from n distinct ones gives C(n, r) = n!/(r!(n − r)!) ways.
In practice
C(5, 2) = 10 — the 20 ordered pairs of Video 515, with each two-way swap counted once instead of twice.
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