✔ 最佳答案
Looks like there are really a lot of people answering this one wrong...
Mr. Wong has one genuine $100 bill and one gift that worths $18. Some young man give Mr. Wong one counterfeit $100 bill, and Mr. Wong used the counterfeit $100 bill to get $100 change from neighbour, and Mr. Wong give the young man the gift and $79 change. So here is what everyone having:
Mr. Wong: 1 genuine $100 bill, $21 change
Young Man: $79 change, $18 gift
Neighbour: 1 counterfeit $100 bill
So later the Neighbour exchange the counterfeit bill with Mr. Wong: so now everyone has
Mr. Wong: 1 counterfeit $100 bill, $21 cash
Young Man: $79 change, $18 gift
Neighbour: 1 $100 genuine bill
Mr. Wong starts with $100 and $18 gift, now he has $21 cash, so he lost $97.