Question

Lisa was choosing between three alternatives: working on her job that pays her $60; writing a term paper which she values at $40; or going out with a friend, which she values at $80. The opportunity cost of going out with her friend is

Answer 1

Opportunity cost is the amount of value that we need to forfeit/sacrifice in order to choose a certain decision.
In this case, the opportunit cost of choosing to go with friends would be:

($40 + $60) - $80

= $ 100  - $ 80

= $20