A hotel uses an external laundry service to provide clean towels. The hotel generates 600 soiled towels a day. The laundry service picks up the soiled towels and replaces them with clean ones at regular intervals. There is a fixed charge of $81 per pickup and delivery service, in addition to the variable cost of $.60 per towel. It costs the hotel $.02 a day to store a soiled towel and $.01 per day to store a clean one. How often should the hotel use the pickup and delivery service?