Problem
You won a free ticket to see a Rihanna concert (assume the ticket has no resale value). Beyonce has a concert the same night, and this represents your next-best alternative activity. Tickets to the Beyonce concert cost $80, and on any particular day, you would be willing to pay up to $100 to attend Beyonce's concert.
Assume that there are no additional costs of attending either show. Based on the information presented here, what is the opportunity cost of attending the Rihanna concert?