Something to help you get started: if S is the power set on R, then for any two randomly selected subsets A and B there are four possibilities: A is a subset of B, B is a subset of A, A is equal to B and A and B are disjoint sets (meaning they have no intersection - for example S= {1,2,3}, A could be {1} and B could be {2,3} - both subsets on S, neither containing the other).

Hopefully that is a step in the right direction.