They pay $30 to the manager and go to their room.

The manager finds out that the room rate is $25 and gives the bellboy $5 to return to the guests.

On the way to the room the bellboy reasons that $5 would be difficult to split among three people

so he pockets $2 and gives $1 to each person.

Now each person paid $10 and got back $1. So they paid $9 each, totaling $27.

The bellboy has another $2, adding up to $29.

Where is the remaining dollar?