I count 15 separate ways... 
  
20 dimes 
15 dimes, 1 half dollar 
15 dimes, 2 quarter 
10 dimes, 2 half dollars 
10 dimes, 4 quarters 
10 dimes, 1 half dollar, 2 quarters 
5 dimes, 3 half dollars 
5 dimes, 2 half dollars, 2 quarters 
5 dimes, 1 half dollar, 4 quarters 
5 dimes, 6 quarters 
8 quarters 
4 half dollars 
6 quarters, 1 half dollar 
4 quarters, 2 half dollars 
2 quarters, 3 half dollars 
  
That's 15