Permutation means arrangement of things while Combination means selection of things. Permutation is used when the order of things is important and combination is used when the order of things has no importance.
Permutation(important) - nPr = n!/(n-r)! n - bigger number r - smaller number
Combination(unimportant) - nCr = n!/(n-r)!r!
Ex 1. A coach must choose five starters from a team of 12 players. How many different ways can the coach choose the starters?
The order is not important so 12C9 = 12!/3!9! = 792
Ex 2. In how many ways can a sorority of 20 members select a president, vice president and treasury, assuming that the same person cannot hold more than one office.
The order is important so 20P3 = 20!/17! = 6840
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