3 guys walk into a hotel and pay $30 for a room.
The clerk realizes he overcharged them $5 so he gives it to the bellhop to take to their room.
Realizing they can't easily split 5 between the three of them they each take one dollar and give the bellhop a $2 tip.
Since they each got $1 back they each paid $9 for room, 3*9 is $27.
$27 plus the $2 tip for the bellhop is $29.
Where did the dollar go?
This was a favorite of my Father's when we were kids.