• 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:
  • ⁿP_r = ^n!/_{(n – r)!}
• The order matters

Combination

• The number ways of selecting r objects from n unlike objects is:
  • ⁿC_r = ^n!/_{r!(n – r)!}
• The order does not matter