how do you get the 30*100 in the cost of goods sold equation?

Aircard Corporation tracks the number of units purchased and sold throughout each accounting period but applies its inventory costing method at the end of each period as if it uses a periodic inventory system. The following are the transactions for the month of July. Calculate the cost of goods available for sale, ending inventory, and cost of goods sold if Aircard uses (b) FIFO, (b) LIFO, or (c) weighted average cost. (Round "Cost per Unit" to 2 decimal places.)