Amy loves collecting coffee mugs and has been collecting them for years. at the drop of a hat, she will launch into a discussion of her collection, the price of mugs, and her plans for purchasing more mugs. her friends say she must have obsessive-compulsive disorder. you disagree. what do you say to her friends when they ask why you disagree?

Atennis club charges a monthly membership fee of $150 and charges it members $10 per hour to use a court. there's also $500 fee to become a member. stella joined the tennis club last spring, and she has been a member for 5 months. she has been playing 10 hours of tennis each month. stella values an hour of her time at $20. during a boring afternoon meeting, stella thinks about reserving a court to work on her serve for an hour tonight. what's stella's opportunity cost of working on her serve for an hour?