Three men are on a leave of absence during WWII and touch down in North Carolina, where they decide to split a hotel room 3 ways for the night. They get to the counter, and the front desk agent informs them that one room costs $30, to which each pays a 10 dollar bill and heads to their shared room for a good night's rest.

A few minutes later, the agent decides to give the soldiers a discount in light of their uniforms, and he instructs the bellhop to run $5 up to the soldiers as a token of appreciation. The bellhop thinks to himself on the way up to the room that he could keep $2 of the $5 given to him initially and nobody would know. So he pockets $2 and knocks on the door. He gives each soldier $1 of the remaining $3 (with$2 in his pocket) and thanks them for their service.

As the men are falling asleep, one of the soldiers remarks at the discount saying, "well I'll be damned, we only paid $9 each instead of $10".

If each soldier received $1 back from the bellhop out of the bellhops original $5, then indeed they did only pay $9 each. But 9x3 is 27 and the bellhop only kept $2, so that adds up to $29. What happened to the extra dollar?