Three men check in to a hotel together at night (Purely business reasons, I assure you). It's late, and the desk worker is tired and grumpy, so he charges them each $10 for the room for a total of $30 (Assume this is back when that was overcharging. Just go with it.)

The next morning, the desk worker comes to his senses and feels bad for overcharging the three gentlemen, and gives the bellhop $5 to take back to them. The bellhop knows that the men don't know about the refund, so he puts $2 in his pocket and gives each of the men $1, so in all, they each only paid $9 for the room.

$9 x 3 is 27, plus the $2 that the bellhop kept is $29. Why is it not $30?