A hotel uses an external laundry service to provide clean towels. The hotel generates 600 dirty towels a day. The laundry service picks up the dirty 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 $0.60 per towel. It costs the hotel $0.02 a day to store a dirty towel and $0.01 per day to store a clean one. how often should the hotel use the pickup-and-delivery service