Final answer:
The opportunity costs for attending the concert are the alternative activities that could have been chosen. Your friend's opportunity cost is the potential knowledge and preparation from studying, while your opportunity cost is the entertainment from watching the sporting event.
Step-by-step explanation:
The relevant opportunity costs for you and your friend for allocating four hours to attending the concert are the alternative activities that you both would have chosen if not attending the concert. In this case, your friend would have chosen to study, while you would have chosen to watch a televised sporting event. The opportunity cost for your friend is the potential knowledge and preparation she could have gained from studying, while your opportunity cost is the entertainment and enjoyment you would have gotten from watching the sporting event. Both of these opportunity costs represent the benefits or values that are lost when choosing one option over another.