You have some limits. For instance, if you were buying only chocolate bars, you couldn’t buy more than 19 without going over your limit. So, you have at most 19 trials to make.

Assume you buy 0 chocolate bars. Then you have 500p left. Divide that by 18. If it works exactly, then there’s your answer. If not, move on.

Buy 1 chocolate bar, you have 474p left. Divide that by 18, same comment.

I honestly don’t know the answer (and I’m not interested enough to find out – *sorry*), but you’ll get there eventually this way. If you get all the way through all of the possibilities for the chocolate bars and never find a value where dividing by 18 works exactly, then the problem is impossible. Do your work carefully, and don’t be afraid of the “impossible” answer. Problems that are impossible show up in mathematics all the time – although I don’t think this is one of them.