Let me see if I got this:

You can't add the $2 that the bellhop dude got to the $27 that the boys paid, you subtract it. The room cost $25 = $27 paid by the boys -minus- $2 involuntary tip.

-or-

you can look at it that they paid $30 -minus- $3 boys pocketed -minus- $2 bellhop pocketed = $25 hotel got.