A tennis 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?