Three backpackers check into a hostel, and pay the clerk $30 for a room.
However, when the manager returns, he notices the clerk has incorrectly overcharged for the room, which should only have cost $25.
The manager then instructs the clerk to refund $5 to the backpackers. The clerk, knowing the difficulty in the backpackers being able to evenly split the fiver, decides to keep $2 and give them $3 back.
The backpackers are happy, having only paid $27 for their room.
But........if they paid $27, and the clerk kept $2, that adds up to $29.
What happened to the other dollar ? :unsu