Note that if you have less than two quarters, it's not possible to get more than 50 cents; and if you have two or more quarters, then you're guaranteed to have more than 50 cents. And you can use as the denominator, just be careful to consider the individual coins as distinct; you could also use as the denominator if you count differently.

Edit: After more reflection, using is overly complicated, because not all combinations are equally likely, hence you would need to use weighted terms somehow..