I had heard it before a while back but I decided to think through it myself and see what was going on. The first thought that occurred to me was that the bellhop's two dollars should be subtracted, just as each of the 1 dollar bills given back to the guests was (to get 27 from 30). Then, I imagined that the guests had not paid with $10 dollars bills, but in 30 ones. The hotel has 30, then each of the three guests is given 1and the bellhop takes two. Here is where the money is:

• Guest 1: 1
• Guest 2: 1
• Guest 3: 1
• Bellhop: 2
• Hotel: 25
• Total: 30

It is basically following all the individual dollars to see where they are and make sure none are missing. Alternately, you can imagine that the price really was 27 in the beginning, they all paid 9, and then the bellhop just stole 2 on the assumption that the owner wouldn't notice.