I do not understand the formula for Target inventory at all.

Nationwide Auto Parts uses a periodic review inventory control system for one of its stock items. The review interval is 8 weeks, and the lead time for receiving the materials ordered from its wholesaler is 5 weeks. Weekly demand is normally distributed, with a mean of 175 units and a standard deviation of 35 units. Refer to the standard normal table for z-values. a. What is the average and the standard deviation of demand during the protection interval? The average demand during the protection interval is 2275 units. (Enter your response as an integer.) The standard deviation of demand is 126.19 units. (Enter your response rounded to two decimal places.) b. If the firm desires 98 percent stockout protection, the target inventory level is units. (Enter your response rounded to the nearest whole number.)