What are npr and ncr? how are they used in probability? . explain your answer and provide some reference links.

The Permutations or nPR is a formula that is used to arrange r things out of n total ones while the Combinations or nCr is a formula that is used to select r things out of n total ones. I hope you are satisfied with my answer and feel free to ask for more 

Apolygon is any 2-dimensional shape formed with straight lines e.g a triangle, square, pentagon, etc