Q:

How to calculate nPr mod m, when m is a prime number?

I need to find out the value of nPr%m.
This is the approach I used.
Find, n!%m, (n-r)!%m and divide them
However, for certain cases, (n-r)!%m is greater than n!%m, so the resultant nPr is 0.
What do I need to do then?

A:

This is more a math question than a programming question, but anyway.
Note that 
n! / (n - r)! = n * (n - 1) * ... * (n - r + 1)

Now for multiplication, 
(a * b * c) % m = (((a * b) % m) * c) % m

i.e. rather than mod ming the entire product, you can mod m the intermediate result of any multiplication of two factors in the product.
I won't provide the full code here, but hopefully this will be enough for you to figure it out.

