A doctor's office has 3 types of workers: physician, nurses, and receptionists. On average, a physician sees each patient for 55 minutes, a nurse spends 35 minutes with each patient, and a receptionist spends 5 minutes with each patient. Suppose there is only one receptionist, whose utilization is 0.8 at current level of demand (you can assume that they are able to meet the demand). What is the minimum number of nurses they must have (to meet the demand)?