I've heard this a few times in person and on the radio. Here is the problem and solution.

Problem
I work in a hotel. Three men come and pay me £100 per room for their stay. I take £300 from them. The manager then tells me that they've overpaid because there's a 3 rooms for £250 offer. The manager gives me £50 to give back to the men. I can't divide £50 by 3 and so give each of them £10 instead. I pocket the £20 remaining. The three men have paid £90 each now, giving £270. There's my £20, which makes £290... where's the other £10 gone from the original £300?

Solution
Well, the answer is in the question... in that the question's wrong. The men, when I've refunded them, have actually paid £270 in total. The hotel has taken £250 from them and my pocketted £20 makes £270. Simple when you know, which I did.