You need to find every combinaison of bills.
ex : $1 + 10$ or 5$+10$
I takes for ever to list all the possibilities so you need to think about probabilities to chose a bill.
For each bill, you can add it to your sum or not; so it gives you 2 possibilities; you do that for each bill and you multipliate each what gives you :
2 * 2 * 2 * 2 = 16 different sums
