please explain how to calculate weighted average

11) A company purchased 500 units for $40 each on January 31. It purchased 200 units for $35 each on 11) February 28. It sold a total of 200 units for $50 each from March 1 through December 31. If the company uses the weighted-average inventory costing method, calculate the cost of ending inventory on December 31. (Assume that the company uses a perpetual inventory system. Round any intermediate calculations two decimal places, and your final answer to the nearest dollar.) A) $19,285 B) $7715 C) $18,750 D) $27,000