I lifted this puzzle from a page by Kevin O Donavan who used it as a very simple but clever way to grow his contact list on Linkedin. It goes as follows:
Three guests check into a hotel room. The receptionist says the bill is €300, so each guest pays €100. Later the receptionist realises the bill should only be €250. To rectify this, he gives the porter €50 to return to the guests. On the way to the room, the porter realises that he cannot divide the money equally. As the guests didn’t know the total of the revised bill, the porter decides to just give each guest €10 and keep €20 for himself. Each guest got €10 back: so now each guest only paid €90; bringing the total paid to €270. The porter has €20. €270 + €20 = €290 so, if the guests originally handed over €300, what happened to the remaining €10?