03 Periodic Inventory System and Inventory Costing Methods E 9. In chronological order, the inventory, purchases, and sales of a single product for a recent month are as follows: Using the periodic inventory system, compute the cost of ending inventory, cost of goods sold, and gross margin. Use the average-cost, FIFO, and LIFO inventory costing methods. Explain the differences in gross margin produced by the threc methods. Round unit costs to cents and totals to dollars