If Sarah has $50 to spend and she decides to buy a new pair of shoes instead of going to a concert, what is the opportunity cost of her decision?

Opportunity cost is what you give up when you make a choice. In Sarah's case, she has $50 and chooses to buy a new pair of shoes instead of going to a concert. The opportunity cost here is the enjoyment and experience she misses out on by not attending the concert. For example, if the concert was something she really wanted to see and would have made her happy, that happiness is what she loses when she buys the shoes. So, while the shoes may be nice, the opportunity cost is the fun and memories she could have had at the concert.