The question no is 5. And the problem lies in calculating additional information 2.
Pardon me for I cannot provide you the question so I will have to give you the link.

http://www.xtremepapers.net/CIE/Cambridge%20O%20Levels/7110%20-%20Principle%20of%20Accounts/7110_s09_qp_2.pdf

It is for 420kb.
It is on Page no 14.

It will be a great help

Use the little thing at the top of your typing space that looks like a tiny globe. That brings up a box you can put a URL in and it will be a clickable link, like this:
http://www.xtremepapers.net/CIE/Cambridge%20O%20Levels/7110%20-%20Principle%20of%20Accounts/7110_s09_qp_2.pdf

Since it has a 2 hour limit (which means in person), I'll assume this is a practice exam and not the real thing.

I suspect they're meaning for you to count the 20% as a purchase cost - i.e. like transportation would be, and the other 80% could go under selling expenses.