Question that proves Singapore University students are dumb dumb

Yesterday a fren sent me a sms about a maths question, supposedly for primary school kids, but 99% of university undergrads got it wrong, wanting to prove it wrong, i ans the sms. I got correct in my first try, haha. I then forwarded the sms to many frens to see if they can get it correct. Out of about 20 people, only 2 got correct, haha, so its not true 99% are dumb dumb.
The tough question is:
A customer went to Mr Goon's shop to buy a gift. The gift cost $18, but Mr Goon is selling at $21. The customer then give Mr Goon a $100 note, having no small change, Mr Goon then exchange the money with his neighbour, Mr Doo. Mr Goon then pay back $79 to the customer. Later Mr Doo found out that the $100 note is fake, having no choice, Mr Goon have to give back $100 to Mr Doo. In this transaction, how much did Mr Goon lost?