VTRMC 1979 Problem 7

Let S be a finite set of non-negative integers such that $| x - y | \in S$ whenever $x , y \in S$ . (a) Give an example of such a set which contains ten elements. (b) If $A$ is a subset of $S$ containing more than two-thirds of the elements of $S$ , prove or disprove that every element of $S$ is the sum or difference of two elements from $A$ .