Suppose Downing Inc. expects to use 6000 units of inventory per year. The cost of placing and receiving an order is $100 per order. Each unit of inventory costs Downing $80. Carrying costs are 20 percent of the purchase price. (a) Calculate the number of units that Downing should order at a time if it is to minimize its total inventory costs. (b) What are Downing's total costs of ordering and carrying inventor for a e the optimal order size calculated in part (a)? How frequent should Downing order