# Combinations

## Introduction

The number of k-combinations of an n-set can be expressed in terms of the number of k-permutations in that set.

- For every k-combination
- there are exactly k! permutations of its elements

k-combination of an n-set
`(number of permutations of n) / (number of permutations of k)`

Why?

## REFERENCES

Combinations. Sal Khan. Khan Academy.