Assuming a perpetual inventory system is used, use the following information to calculate cost of goods sold

Question:

Assuming a perpetual inventory system is used, use the following information to calculate cost of goods sold on an average-cost basis.
Dec. 1 Beginning inventory 50 units @ $22
9 Purchases 50 units @ $24
17 Sales 25 units
22 Purchases 75 units @ $27
27 Sales 40 units