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Permutation and Combination

  • Basic counting principle: To find the number of ways of performing several tasks in succession, multiply the number of ways in which each task can be performed:
    • E.g. 5 × 4 × 3 × 2 × 1
  • Factorial; n! = n × (n -1) × (n – 2) … × 3 × 2 × 1
    • NOTE: 0! = 1
Permutation
  • The number of ordered arrangements of r objects taken from n unlike objects is:
    • nPr = n!/(n – r)!
  • The order matters
Combination
  • The number ways of selecting r objects from n unlike objects is:
    • nCr = n!/r!(n – r)!
  • The order does not matter
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