The Missing Dollar Riddle

3 friends pay $30 for a room, $10 each. The clerk realizes the room costs only $25 and gives $5 to the bellhop to return. The bellhop keeps $2 and gives each friend $1 back. So each friend paid $9, and 3 times 9 is 27. Add the bellhop's $2 and you get 29. Where is the missing dollar?

Source: A classic puzzle, retold by INFINITE SUM and fact-checked against our curated notes