LMAO!
KY well....20-40 = 30.[8D]

Simplest way of explaining is in bold.
The bell hops 2 dollars is INCLUDED in the 27. 3x9 does = 27, but you cant add 2, because it is already figured in. you have to add in the 3 dollars they got back to the 27 they spent to get back to the original 30.