Weighted Average Cost Flow Method Under Perpetual Inventory System The following units of a particular item were available for sale during the calendar year: Jan. 1 Inventory 4,000 units at $20 Apr. 19 Sale 2,500 units June 30 Purchase 6,000 units at $24 Sept. 2 Sale 4,500 units Nov. 15 Purchase 1,000 units at $25 The firm uses the weighted average cost method with a perpetual inventory system.