A B-Buy retail store purchases household security systems from a distributor for resale. For an upcoming promotion, the retailer needs to determine the best order size for a one-time purchase. One particular brand of household security system will have a special sale price of $350. The retailer estimates the probabilities for selling the system follow a normal distribution with a mean of 80 and a standard deviation of 20. The system can be purchased from the distributor for $250 each, but there is a restocking charge of 20 percent of the purchase price for the return to the distributor of any unsold systems. Please analyze this problem by determining the unit profit (cost of underestimating the demand Cu), the unit loss (cost of overestimating the demand Co), and associated service level to achieve, and then suggest the optimal size of purchase order for the retailer. [No rounding to integer is necessary to show your final answer]