Can you please explain how to compute the subtotal average costs? That's the only part I do not understand. Thank you.

Required information [The following information applies to the questions displayed below.] Ferris Company began January with 7,000 units of its principal product. The cost of each unit is $9. Merchandise transactions for the month of January are as follows: Date of Purchase Jan. 10 Jan. 18 Totals Units 6,000 7,000 13,000 Purchases Unit Cost* $ 10 11 Total Cost $ 60,000 77,000 137,000 Includes purchase price and cost of freight. Sales Date of Sale Jan. 5 Jan. 12 Jan. 20 Units 3,000 1,000 4,000 8,000 Total 12,000 units were on hand at the end of the month. 5. Calculate January's ending inventory and cost of goods sold for the month using Average cost, perpetual system. (Round average cost per unit to 4 decimal places. Enter sales with a negative sign.) Perpetual Average Beginning Inventory Sale - January 5 Subtotal Average Cost Purchase - January 10 Subtotal Average Cost Sale - January 12 Subtotal Average Cost Purchase - January 18 Subtotal Average Cost Sale - January 20 Total Inventory on hand Cost of Goods Sold # of units Cost per Inventory # of units Avg.Cost Cost of unit Value sold per unit Goods Sold 7,000 9.0000 $ 63,000 (3,000) 9.0000 (27,000) 3,000 $ 9.0000 $ 27,000 4,000 9.0000 36,000 6,000 10.0000 60,000 10,000 10.00001 96,000 (1,000) 10.0000 (10,000) 1,000 $ 10.0000 $ 10,000 9,000 10.0000 86,000 7,000 11.0000 77,000 16,000 10.0000 163,000 (4,000) 10.0000 (40,000) 4,000 $ 10.0000 40,000 12,000 $ 123,000 8,000 $ 77,000