S has an upper bound. that is, all its elements are less than or equal to b. so what i did was to construct the set of ALL elements of natural numbers that are less than or equal to b. showing S is a subset is not that difficult, for assume x is some element in S, then that means it is a natural number less than or equal to b, but then, that means x is in B, since B is the set of ALL such elements. thus, S is a subset of B.