[SixOfSpades]

The secret behind the trick is the way it's worded--making you add and subtract at the same time. Here, I'll give a simpler example: Lay your hands on your desk, fingers spread. How many fingers do you have? Ten. Let's count them to make sure; going from your left pinky to your right pinky we'll number your fingers in order from 1 to 10. Now this is the tricky part, adding and subtracting at the same time: Let's count your fingers backwards, from your right pinky down to your right thumb: 10, 9, 8, 7, 6. But we know that your left hand has 5 fingers, so we can simply add that to the 6 we got for the right hand, giving a total of 11 fingers.

The salesmen did indeed give up $27 for the room: The $25 that are rightfully in the till, and 1 each for the clerk & the bellboy, just as 250 said. You don't add $27+clerk+bellboy to get $29, you add $27+salesman+salesman+salesman to get $30.