问题如下：
Jason Williams purchased 500 shares of a company at $32 per share. The stock was bought on 75 percent margin. One month later, Williams had to pay interest on the amount borrowed at a rate of 2 percent per month. At that time, Williams received a dividend of $0.50 per share. Immediately after that he sold the shares at $28 per share. He paid commissions of $10 on the purchase and $10 on the sale of the stock. What was the rate of return on this investment for the one-month period?
选项：
A.−12.5 percent.
B.–15.4 percent.
C.–50.1 percent.
解释：
B is correct.
The return is –15.4 percent.
Total cost of the purchase = $16,000 = 500 × $32
Equity invested = $12,000 = 0.75 × $16,000
Amount borrowed = $4,000 = 16,000 – 12,000
Interest paid at month end = $80 = 0.02 × $4,000
Dividend received at month end = $250 = 500 × $0.50
Proceeds on stock sale = $14,000 = 500 × $28
Total commissions paid = $20 = $10 + $10
Net gain/loss = −$1,850 = −16,000 − 80 + 250 + 14,000 − 20
Initial investment including commission on purchase = $12,010
Return = −15.4% = −$1,850/$12,010
请问为什么不是除以自有资金4010？
