For the money exchanged: The true bill was $25 and the bellboy gave each of the three guests a $1 back. Now, the total exchanged is $28. And the bellboy has $2. That equals the $30.
For the intangible part: You can't figure in that each guest paid exactly $9 for the room because the room bill was $25 which can't be divided between 3 people evenly. Each person would have paid exactly $8.33333333333333 each.
Take the $8 x's 3 people = $24
Plus $3 they were given back = $3
.33333333333333 x's 3 people = $1 <--- the missing dollar
and the bellboy has $2
Total = $30
- Hope I did this right