Please help me with explanation

how to calculate the cost of goods available for sale and ending inventory in this case?

Exercise 6-13 Deala Ltd. ("DL") is a retailer of office equipment. The company uses a periodic inventory system and began October with 2,000 units with a total cost of $40,000. During the month of October, DL had the following inventory-related transactions: Date Explanation Units Unit Cost Total Cost Oct. 9 Purchase 5,000 $21 $105,000 12 Purchases 4,000 20.5 82,000 17 Sale (@ $40/unit) (5,000) 25 Purchase 4,000 20.8 83,200 28 Sale (@ $40/unit) (6,000) x Your answer is incorrect. Try again. Determine the cost of goods available for sale for the month. (Round answers to the nearest whole dollar, e.g. 5,275.) FIFO Average Cost of goods available for sale 270200 270200 x Your answer is incorrect. Try again. Calculate ending inventory at October 31 and cost of goods sold for the month assuming that DL used (1) FIFO and (2) average cost. ( Round average cost per unit to 2 decimal places, e.g. 50.25 and all other answers to the nearest whole dollar, e.g. 5,275.) FIFO Average x Cost of ending inventory 41600 41600 Cost of goods sold 228600 228600