# Summary

- The number of permutations of n different things taken r at a time is denoted as
^{n}P_{r} - n factorial is the product of n, n - 1, n - 2, ...1
- Permutation means counting the number of ways in which some or all objects at a time are arranged, whereas combination means counting the number of ways in which some or all objects at a time are selected
- The number of combination of n different objects taken r at a time is denoted as

nC_{r}.