NO.PZ2016012101000140
问题如下：
Cinnamon Corp. started business in 2007 and uses the weighted average cost method. During 2007, it purchased 45,000 units of inventory at €10 each and sold 40,000 units for €20 each. In 2008, it purchased another 50,000 units at €11 each and sold 45,000 units for €22 each. Its 2008 cost of sales (€ thousands) was closest to:(assume the companies use a periodic inventory system)
选项：
A.€490.
B.€491.
C.€495.
解释：
B is correct.
Cinnamon uses the weighted average cost method, so in 2008, 5,000 units of inventory were 2007 units at €10 each and 50,000 were 2008 purchases at €11. The weighted average cost of inventory during 2008 was thus (5,000 ×10) + (50,000×11) = 50,000 + 550,000 =€600,000, and the weighted average cost was approximately €10.91 =€600,000/55,000. Cost of sales was €10.91×45,000, which is approximately €490,950.
解析：2007年买的存货没全卖掉，2008年年初库存还有5000件，每件单价10块，2008年又采购了50000件，每件11块，加起来总成本=50000+550000=600000，再除以总件数55000，加权平均成本=600000/55000=10.91，这是2008年每件存货的成本，问的是2008年的COGS，2008年卖掉了45000件，COGS=45000*10.91=490950，用thousand作单位就是491。
之前看有一个答案是在这样的：
算2008的平均成本，就是用这一年每一次购买的成本加权平均
2007年买入了45000，卖出了40000，所以剩下5000，这5000就是2018的初始成本，即5000*10
然后2008年又买入了50000，每个11块，所以就是50000*11
所以平均就是（5000*10+50000*11）/（5000+50000）
请问（5000*10+50000*11）/（5000+50000）这个公式怎么理解？为什么不是（5000*10+40000*11）/（45000）？
