5.7 It takes a phlebotomist 15 minutes to complete a blood draw. The supplies for each draw cost $4, and the phlebotomist earns $20 per hour. The phlebotomy lab designed to accommodate 20,000 draws per year. Its rent is $80,000 per year. What are the average and incremental costs of a blood draw when the volume is 20,000? 10,000? What principle does your calculation illustrate?