R stands for how many things you are choosing. N-r where n represents the number of items and r represents the. The coefficient will be the total number of possible combinations.
In other words choosing 3 balls out of 16 or choosing 13 balls out of 16 have the same number of combinations.
For an in-depth explanation of the formulas please visit Combinations and Permutations. Which is the same thing as n k n-k. Or we could do it this way. Online calculator combinations without repetition.