How do we find weighted average? Please show your work so I can understand it, thank you.

! Required information [The following information applies to the questions displayed below.] In its first month of operations, Literacy for the Illiterate opened a new bookstore and bought merchandise in the following order: (1) 460 units at $5 on January 1, (2) 620 units at $6 on January 8, and (3) 920 units at $7 on January 29. Assume 1,200 units are on hand at the end of the month, calculate the cost of goods available for sale, ending inventory, and cost of goods sold under the weighted average cost flow assumptions. Assume a periodic inventory system is used. (Round "Cost per Unit" to 2 decimal places.) Weighted Average Cost $ 12,460 Cost of Goods Available for Sale Ending Inventory Cost of Goods Sold