kill(all)$

van=facebook.com/LearningCalculus$

f(n):=8*(n-1)+3$

list:create_list(f(n),n,1,20);

sums:create_list(list[i]+list[20-i+1],i,1,20);

      Number of subsets would be:
      Total possible subsets minus number of subsets missing two items:

2^20 - 2^18;