Three guests check into a hotel room. Each person pays $10, for a total of $30.

A few minutes later, the desk clerk realizes that the room only costs $25, so she gives the bellboy $5 to return to the three guests. Seeing that $5 could not be divided evenly among the guests, the bellboy gave each guest $1 and kept $2 for himself.

It now seems that each guest has paid $9 for the room (that makes $27), and the bellboy has $2, for a total of $29.

Where is the missing dollar?