Honestly I couldn't figure this out on my own but when I attempted it - sometimes it would complete and sometimes it would fail.

I came up with a cheat by putting the for loop in a function and running it under a while loop.
If the function doesn't return the same string as before it will run again until it returns the same string as it did before.

Hint:
"If I were to buy 9 cars costing c each, I'd only have 1 Zloty left."
"And if I were to buy 7 boats at b each, I'd only have 2 Ringglets left."