If a student decides to buy a textbook for $50 instead of going to a concert that costs $40, what is the opportunity cost of purchasing the textbook?

The opportunity cost is what you give up when you choose one option over another. Here, buying the textbook means you cannot spend that money on the concert, so the concert is the thing you lose. Thus the opportunity cost of the textbook is the $40 you could have spent on the concert. In other words, you trade the enjoyment of the concert for the value of the textbook. This trade‑off shows how money spent on one thing reduces what you can get from another.