The only digit that can be 1/5 the last would be 1 if the last were 5 (because 2/10 doesn't work, 10 isn't a single digit and they need to be whole).

From there, digits 2 and 3 are the last (5) multiplied by 3. So: 1155. Add those up: 1 + 1 + 5 + 5 = 12.

I think. :)