一題攸關成本 - 會計

The most recent monthly income statement for Benner Store is given below:
total store A store B

sales 1,000,000 400,000 600,000
variable expenses 580,000 160,000 420,000
___________ __________ _________
CM 420,000 240,000 180,000
traceable fixed expenses 300,000 100,000 200,000
___________ __________ _________
store segment margin 120,000 140,000 (20,000)
common fixed expenses 50,000 20,000 30,000
___________ __________ _________
net operating income 70,000 120,000 (50,000)

Due to its poor showing, consideration is being given to closing store B.
studies show that if store B is closed, one-fourth of its traceable fixed
expenses will continue unchanged. the studies also show that closing store B
would result in a 10 percent decrease in sales in store A. the company
allocates common fixed expense to the stores on the basis of sales dollars.
Require: compute the overall increase or decrease in the company's operating
income if store B is closed. [98成大會研]

答：營業利益減少6,000

我的想法：
可免成本：420,000+200,000*3/4=570,000
減少利益：600,000+400,000*10%=640,000
570,000-640,000=(70,000) ←營業利益減少70,000

有人可以告訴我，我錯在哪裡嗎？謝謝！

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