off topic math question

[Mindsspring]

Does anyone know the math the name of the math puzzler that goes like this:   3 guys go to a hotel, the manager says the room is $30. Each guy puts in $10 then go to sleep.   The next manager decides the room is really only $25, but decides to keep $2 for himself.   He then hands each guy back $1 each.   The Question is if each guy paid $9 for the room, and 3*9=27 +2(the manager took) what happens to the other dollar, since 27+2=$29?   I only ask since I belive this is a math riddle and wanted to know the name.   Nigel