math challenge

Three sports fans rent a box for a ball game. It costs $300, and they give the ticket clerk $100 each. Shortly after, the clerk discovers that their box only costs $250, so he goes to return their money. He discovers that the best he has for change is five $10 bills, so he can’t divide the money equally in any case. He decides to give them back $10 each, and keep $20 for himself. After he does that, the fans have each spent $90 for their share of the box, and the clerk has $20. Since three times $90 is $270, and the $20 the clerk kept makes $290, where has the extra $10 gone?