each sale ent the data in the form illustrated in Exhibit 6. EX 7-9 Weighted average cost flow method under perpetual inventory system oB.3 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