Keep in mind that this riddle is more than 40 years old. A time when a $30 motel room was a possibility:
Three guys on the road stopped at a motel to spend the night. The clerk told them the room was $30. They each coughed up $10. After they left the clerk realized he had over-charged them. The room was actually only $25. Unsure of how to split the $5 refund three ways he decided to give each of the three customers back $1 each and just pocket the remaining $2 himself.
So...each customer originally spent $10 and then received a $1 refund, That means they each spent $9 for a total of $27. The clerk kept $2. The $27 the customers paid and the $2 the clerk kept totals $29. What happened to the missing $1?