If you do not use any $1's, you can use from 0 through 10, inclusive, 20c's, and
the rest, if any, in 10c's.  That's 11 ways.

If you use 1 $1, you can use from 0 through 5, inclusive, 20c's, and the rest,
if any, in 10c's.  That's 6 ways.

There is just 1 other way.  You can use 2 $1's.  

Total = 11+6+1 = 18 ways.

Edwin