please answer with formula

Problem 3 (16 points): Shoe-n-co places an order for a batch of athletic shoes every time the number of such shoes at the warehouse falls below a specified target. Average demand for the shoes is 1,800 units per week, the ordering cost is $80 per order, and the holding cost is $40 per unit per year (please assume 50 weeks in a year). a. (4 pts.): What is the optimal size of the order? b. (4 pts.): Suppose that the demand during lead time is normally distributed with a mean of 7,200 units and a standard deviation of 900 units. If the desired service level is 95%, what is the reorder point (ROP)? c. (4 pts.): What is the annual cost of ordering and holding? d. (4 pts.): What is the standard deviation of the weekly demand