Let's combine a and b to single element that we call *. Then our set will become A={0,1,2,3,*}. Since this set has 5 elements it has 32 subsets (2⁵=32). Out of these 32 sets, there are 16 sets that doesn't contain *. (If we delete *, there remains a set containing 4 elements which has 2⁴=16 subsets). A subset will either contain * or it will not contain *. Thus 32-16=16 sets contain *, namely a and b.