The 123 Accounting Company sells accounting videos. The following information has been extracted from the records of 123 Accounting Co.

January 1: Opening Inventory is 60 units @$10/each

January 10: Bought 100 units @$11 each

January 15: Bought 100 units @$13 each

January 20: Sold 220 units @$20 each

January 25: Bought 40 units @ $15 each

If 123 Accounting uses the weighted average method for inventory valuation and the periodic system then the unit cost charged to cost of sales when the sale is made would be:

(HOW CAN I CACULATED THIS QUESTION)