How Many Ways Can You Make Change For A 50 Bill Using 5, 10, 20 Bills?

I count 12 ways. There may be more.
2*20 + 10
2*20 + 2*5
20 + 3*10
20 + 2*10 + 2*5
20 + 10 + 4*5
20 + 6*5
5*10 (five $10 bills)
4*10 + 2*5
3*10 + 4*5
2*10 + 6*5
10 + 8*5
10*5 (ten $5 bills)