Combinatorics is a branch of mathematics with applications in many fields, such as physics, economics, computer programming, and many others. In particular, probability theory is one of the fields that makes heavy use of combinatorics in a wide variety of contexts. For example, when calculating probabilities, you often need to know the number of possible orderings or groupings of certain events, outcomes of experiments, or generally any kinds of objects.

Here’s what I’m talking about.