Kid 1 pays $10.
Kid 2 pays $10.
Kid 3 pays $10.

The hotel owner has $30.
The hotel owner returns $5.
The hotel owner now has $25. (The cost of the room)

The bell boy keeps $2.
The bell boy gives each kid $1.

Kid 1 only had to pay $9.
Kid 2 only had to pay $9.
Kid 3 only had to pay $9.

Each kid has paid $9, the bell boy has $2, and owner has his $25.

Don't try to add up to 30.
You have to subtract down to 25.