Three friends went out for drinks. The waiter brought them a check for $30, so each one of them paid $10. When the waiter took the cash, he realized he had made a mistake, and the check was for $25 instead. When he gave their change back, each friend got a dollar and they left the remaining two dollars as a tip. Therefore, each customer paid $9; multiplied by 3 this equals $27; plus $2 for the tip equals $29. Where is the remaining dollar?