Here is a hint for anybody who wants to try it:
You can only make the sum of 10 in four different ways as follows:
6 + 2 + 2 = 10 - permutations =3! / 2! =3
5 + 3 + 2 = 10 - .................... =3! = 6
4 + 4 + 2 = 10 - .................... =3! / 2! =3
4 + 3 + 3 = 10 - .................... =3! / 2! =3
And you can only make the sum of 20 in eight different ways as follows:
9 + 9 + 2 = 20 - permutations = 3! / 2! =3
9 + 8 + 3 = 20 - ..................... = 3! =6
9 + 7 + 4 = 20 - ..................... = 3! =6
9 + 6 + 5 = 20 - ..................... = 3! =6
8 + 8 + 4 = 20 - ..................... = 3! / 2! =3
8 + 7 + 5 = 20 - ..................... = 3! =6
8 + 6 + 6 = 20 - ..................... =3! / 2! =3
7 + 7 + 6 = 20 - ..................... =3! / 2! =3
That is as far as I go!!!.