Hello. I read the tutorial of the problem Roman Digits but can't understand it. My thought is we need to count different k such that there exist a, b, c, d with the following conditions:

(i)a + 5b + 10c + 50d = k

(ii)a + b + c + d = n

If we replace a from (ii) we get 4b + 9c + 49d = k - n. Can't continue.

Any helps are appreciated.