I don't konw why the average cost is wrong?

Inventory Costing Methoos Anderson's Department Store has the following data for inventory, purchases, and sales of merchandise for December: Purchase Price Sale Price Activity Beginning inventory Purchase 1, Dec. 2 Purchase 2, Dec. 5 Sale 1, Dec. 7 Sale 2, Dec. 10 Purchase 3, Dec. 12 Sale 3, Dec. 14 Anderson's uses a perpetual inventory system. All purchases and sales were for cash Required: Units (per unit) (per unit) 10 $8.00 8.80 9.05 26 19 25 12 20 $20.00 20.00 9.80 20.00