Gregory is considering attending a concert with a ticket price of​ $40. He estimates that the cost of driving to the concert and parking there will add an additional​ $20. In order to attend the​ concert, Gregory will have to take time off from his partminustime job. He estimates that he will lose 5 hours at​ work, at a wage of​ $8 per hour.​ Gregory's opportunity cost of attending the concert equals.