I forget if you want to use permutations or combinations here... it will be one over one of the following formulae:
n is the total number of items, r is the number of those items you want.
Combination (order does not matter):
n! / [ ( n - r )! * r! ]
Permutation (order matters):
n! / (n - r)!
So for probability, you have either
[ ( n - r )! * r! ] / n!
or
(n - r)! / n!
Depending on which is the one you want.
