] For product SKU B5392 at SuperMart, the order quantity is set at 200 units with each order. The daily demand for this item follows a normal distribution, having a mean of 18 units and a standard deviation of 6. The lead time required for the item to arrive after placing an order is fixed at six days. The holding cost per unit per year is $8. To meet a 98% service level, the store's management has decided to take action. . Determine the standard deviation of demand during the lead time