combination formula
https://www.aplustopper.com/combination-formula-ncr/
https://sites.google.com/site/aplustoppernotes/combination-formula
http://cbsetuts.blogspot.in/2018/01/combination-formula.html
Definition:
Each of the different groups or selections which can be formed by taking some or all of a number of objects, irrespective of their arrangements, is called a combination.
Notation: The number of all combinations of n things, taken r at a time is denoted by
nCr is always a natural number.
Read more about combination formula