You have received two good responses showing the calculation based on the definition of nCr.
If you have to do a lot of calculations of nCr numbers, you will be wasting a lot of time if you go back to the definition of nCr every time.
The expression to evaluate nCr is a fraction with a product of r numbers in both numerator and denominator; the numbers in the numerator start with n and count down; the numbers in the denominator start with r and count down.
