Final answer:
The opportunity cost of Margaret's decision not to sell the painting is $20,000.
Step-by-step explanation:
The opportunity cost of Margaret's decision is $20,000.
Opportunity cost refers to the value of the next best alternative that is forgone when making a decision. In this case, Margaret had the opportunity to sell the painting for $25,000, but she chose not to. The $20,000 represents the value of the painting that Margaret could have received if she had sold it.