The word derangement in simple words means any change in the existing order of things. Mathematically, derangement refers to the permutation consisting of elements of a set in which the elements don't exist in their respective usual positions.

This section contains detail theory on combination or selection. It also helps you to understand the formula use in combination and when and where to use combination as well as what are the difference between combination and permutation.

The number of circular permutations of n different things is (n-1)! In this arrangement anti-clockwise and clockwise order of arrangement are considering as distinct.

Each of the different arrangements which can be made by taking some or all of a number of given things or objects at a time is called a permutation.