There is no formula.  There are four cases:

1. Use no 10 cent or 5 cent coins:

There are 6 ways, for we can use from 0 through 5  2-cent coins and
the rest, if any, in 1 cent coins.

2. Use no 10-cent coin and 1 5 cent coin.

There are 3 ways, for we can use from 0 through 2  2-cent coins and
the rest in 1-cent coins.

3. Use no 10-cent coin and 2 5-cent coins.

There is only one way, 2 5-cent coins.

4. Use 1 10-cent coin.

Also just 1 way.

Total: 6+3+1+1 = 11 ways.  

Edwin