Demand for vanilla ice cream at a small ice cream shop can be approximated by a Normal distribution with a mean of 21 liters per week and a standard deviation of 3.5 litres per week. The ice cream is purchased from an ice cream producer. The store manager desires a lead time service level of 90 percent. He has estimated the holding cost per litre to be $0.1 per year and the cost of order to be $5. Lead time from the producer is two days. The store is open seven days a week. Find optimum order quantity.