If you don't use any 15c stamps, you can put from
0 through 4 10c stamps and the rest in 5c stamps.

That's 5 ways.

If you use exactly 1 15c stamp, you have 20c left,
which you can get by putting put from 0 through 3 
10c stamps and the rest, if any, in 5c stamps.

That's 4 ways.

If you use exactly 2 15c stamp, you have 15c left,
which you can get by putting put from 0 - 1 10c 
stamps and the rest in 5c stamps.

That's 2 ways

Or you can use 3 15c stamps.

That's 1 more way.

Total:  5 + 4 + 2 + 1 = 12 ways.

Edwin