Suppose you won a free ticket to see EXO concert, and assume that it has no resale value. Assume that TFBOYS is performing on the same night and it's the most attractive alternative. If you don't go to see EXO, you will see TFBOYS. If you don't see TFBOYS. you will see EXO. Now, tickets to see TFBOYS costs about $40, but you're willing to pay $50 to see them. And there's no other cost of seeing each performers. Based on above information, what would be the opportunity cost of seeing EXO?