#### 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:
^{n}P_{r}=^{n!}/_{(n – r)!}

- The order matters

**Combination**

- The number ways of selecting r objects from n unlike objects is:
^{n}C_{r}=^{n!}/_{r!(n – r)!}

- The order does not matter