Answer:
The opportunity cost is the cost that is generated by selecting some other alternative. The opportunity cost indicates the value or activity that is foregone to do something else.
In our case, me and my friend decided to go on a concert and concert ticket price is $100.
There are other three alternatives available for me and my friend other than concert:
(i) purchase a textbook worth of $100.
(ii) meal at a highly rated local restaurant
(iii) internet movie downloads
It was given that my next best alternative to the concert is internet movie downloads and my friend's next best alternative to the concert is meal at a highly rated local restaurant.
Hence,
The opportunity cost for me and my friend of the concert tickets that we purchased are internet movie downloads and meal at a highly rated local restaurant, respectively.