L1 問題請教 - 金融分析師

經濟學中
opportunity cost = explicit cost + implicit cost
我想請問一下
教科書V2 P94中有提及 economic depreciation 是包含在 implicit cost 裡面

但我做到一個例題如下：

Total revenue $300,000

Value of buildings and machinery
At the beginning of the year $300,000
At the end of the year $280,000

Cost of raw materials $100,000
Wages paid during the year $50,000
Normal profit for the year $40,000

然它給的解答是

opportunity cost = 100,000+50,000+40,000=190,000
economic depreciation = 300,000-280,000=20,000
economic profit=Total revenue-opportunity cost-economic depreciation
=300,000-190,000-20,000=90,000

為什麼當中的economic depreciation:$20,000要再多減一次？

不知道是不是我哪裡解讀錯誤？
以上是我的問題，先謝謝大家幫忙。


--