I have the answer to this question. Can you explain those 3000 and 4000 and their unit costs?

Altira Corporation provides the following information related to its merchandise inventory during the month of August 2021: 3 Aug. 1 Inventory on hand-2,000 units; cost $5.30 each. 8 Purchased 8,000 units for $5.50 each. 14 Sold 6,000 units for $12.00 each. 18 Purchased 6,000 units for $5.60 each. 25 Sold 7,000 units for $11.00 each. 28 Purchased 4,000 units for $5.80 each. 31 Inventory on hand7,000 units. Required: Using calculations based on a periodic inventory system, determine the inventory balance Altira would report in its August 31, 2021, balance sheet and the cost of goods sold it would report in its August 2021 income state- ment using each of the following cost flow methods: 1. First-in, first-out (FIFO) First-in, first-out (FIFO) Cost of goods available for sale (20,000 units) Less: Ending inventory (determined below) Cost of goods sold $111,400 _(40,000) $ 71,400 Cost of ending inventory: Date of purchase Units August 18 3,000 August 28 4,000 Total Unit cost $5.60 $5.80 Total cost $16,800 23,200 $40,000