48. INVENTORY An electronics firm uses 600 cases of components each year. Each case costs $1,000. The cost of storing one case for a year is 90 cents, and the ordering fee is $30 per shipment. How many cases should the firm order each time to keep total cost at a minimum? (Assume that the components are used at a constant rate throughout the year and that each shipment arrives just as the
preceding shipment is being used up.)​