Final answer:
The opportunity cost of playing paintball instead of attending a concert for which the student already has a $30 nonrefundable ticket is at least $30, because the enjoyment and value of the concert is the next best alternative foregone.
Step-by-step explanation:
The student asks about the opportunity cost of playing paintball when they already have a nonrefundable concert ticket. Opportunity cost represents the value of the next best alternative given up when a decision is made. Here, considering the concert ticket's cost is sunk since it cannot be reclaimed, the opportunity cost of playing paintball is the enjoyment and value that would've been gained from attending the concert. Since the question doesn't specify the value the student places on the concert, it would typically be assumed that the student values the concert at least at the price of the ticket, which is $30. Thus, the opportunity cost in dollars would then be at least $30.