C(x,y) represents the number of combinations of x objects taken y at a time. The formula is:
C(x,y) = x!/y!/(x-y)!
If you have 20 objects, you pull two and you want to know the probability that they will be less than 15, here is the logic. You have 15 "good" balls you can pull. The number of ways to pull two balls just from those 15 is C(15,2) = 15*14/2 = 105. The total number of ways you can pull two balls from 20 balls is C(20,2) = 20*19/2 = 190. That makes the probability 105/190.