Three friends check into a hotel room that costs $30. They each contribute $10, handing $30 to the hotel clerk. Later, the hotel clerk realizes there was a mistake and the room only cost $25. The hotel clerk gives $5 to the bellboy and asks him to return it to the guests. The bellboy, however, decides to keep $2 for himself and gives $1 back to each guest.

Now, each guest has paid $9 (a total of $27), and the bellboy has kept $2, which adds up to $29. What happened to the missing dollar?

The confusion in this brain teaser arises from the way the problem is presented. The guests collectively paid $27 ($9 each), of which $25 goes to cover the actual cost of the room, and the remaining $2 is kept by the bellboy. So, there is no missing dollar.

The fallacy lies in adding the $2 the bellboy kept to the $27 paid by the guests, instead of subtracting it. The guests paid a total of $27, which includes the $25 for the room and the $2 the bellboy kept. Adding the $2 back to the $27 creates the illusion of a missing dollar, but in reality, the math works out correctly.