Explain the concept of opportunity cost?

For spring break, Melanie will either stay home or go to Daytona Beach. At home, Melanie pays $20 for food and earns $100 a day at her job. At Daytona Beach, Melanie will stay with friends and so had no lodging costs. She will pay $40 per day for food. In terms of dollars, Melanie's opportunity cost per day of going to Daytona Beach is how much