请问如果用weighted计算这题那该怎么计算？

问题如下：

A firm that prepares its financial statements according to US GAAP and uses a periodic inventory system had the following transactions during the year:

选项：

A.$5,890 using weighted average.

B.$4,080 using LIFO.

C.$3,850 using FIFO.

解释：

C is correct. Under FIFO, the oldest units are sold first, thus for the six units sold FIFO, cost of sales is $3,850, as follows: 1 unit at $600 + 5 units at 650 = $3,850.

A is incorrect. It is the cost of goods available for sale not the cost of goods sold: 1 unit at $600 + 5 units at 650 + 3 units at 680 = $5,890.

B is incorrect. It used the $680 most recent cost for all 6,000 units sold: 6 × $680 = $4,080.