@Sven Groot: This riddle is dependent on the reader's falling for the bad math that's set up by the question:
This meant that the 3 men each paid $9 for the room, which is a total of $27 add the $2 that the bellboy kept = $29. Where is the other dollar?
It's as silly as this riddle:
How do you pronounce the capital of the State of Kentucky? Loo-ee-vil or Loo-IS-vil?
Of course, the answer to that is "Frankfort" (pronounced "frank-furt").
If the question didn't try to set up the math for you, you'd think of the $2 as the tip and automatically add it to the bill ($25). So you end up with the total being $27 and no math confusion at all.