Examples on counting combinations
4.3
How many 2-element subsets does the set {a,b,c,d,e} have?
5! 5! 5 ?4 ?3 ?2 ?1 120
C(5,2) = ---------- = --------- = ---------------------- = ----- = 10
2!(5-2)! 2! ? 3! (2 ?1) ? (3 ?2 ?1) 12
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