Combinatorics is the branch of mathematics
that deals with determining the number
s to arrange
things, the number of subset
s of a set
that have a certain property, etc.
The two fundamental concepts in combinatorics are permutations and combinations.
A permutation is an arrangement of a set of objects, or specifically an arrangement of a subset of a certain size of some set of objects.
A combination is a subset of a certain size of some set of objects, without regard to their order.
These concepts are inherent to some kinds of probability problems.