This is one of my favorite brainteasers of its type because of its simplicity. You might have realized that the bellboy's $2 and the men's $27 don't need to add up to $30. But do you know exactly why not? And why you fell for it?
The secret to the puzzle is that there are two equations, not one. One deals with cash flow; the other, with the physical legal tender. The puzzle works by getting you to mix the two.
Cash flow: the men are -$27, and bellboy is +$2, and
the proprietor is +$25, adding up to zero. That's one equation.
Legal tender: the men now have $3, the bellboy has $2, and the proprietor has $25, adding up to $30. That's the other.
An oldie but a goodie!