A sandwich manufacturer must order the food materials that make up the sandwiches three days before they are needed by the factory. When these raw materials arrive, they are in stock for, on average, half a day before they are used. The throughput time for a batch of sandwiches is, on average, one hour. They are then stored in a temperature- controlled environment for an average of two hours, after which they are loaded onto temperature-controlled trucks and delivered to customers, which takes on average six hours. What is the P:D ratio for this operation?