Atennis club charges a monthly membership fee of $150 and charges it members $10 per hour to use a court. there is 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 is stella's opportunity cost of working on her serve for an hour?