Branch of mathematics that deals with ideas and methods for counting. especially in complex situations.
There are easy ways to count and also complicated ways to count without counting at all.
An example of physical counting is through using a Tree Diagram and counting branches.
Combinatorics with Cases and Permutations
On occasion, you may be faced with a problem with specific restrictions, that also requires permutations to be able to solve. https://www.khanacademy.org/math/precalculus/x9e81a4f98389efdf:prob-comb/x9e81a4f98389efdf:prob-combinatorics-precalc/v/probability-permutations-combinations-taste-testing