问题如下：
When the stock price of PZ company was $30, Tim purchased 300 shares. He bought the stocks on 50 percent margin. One month later, Tim received a dividend of $1 per share and had to pay interest on the amount borrowed at a rate of 4% per month. After received dividends, Tim sold the shares at $25 per share immediately. The commissions for both the purchase and the sale of the stocks are $5. Which of the following statements is correct?
选项：
A.The rate of return on this investment for the one-month period is -30.71%.
B.The rate of return on this investment for the one-month period is –30.85%.
C.The rate of return on this investment for the one-month period is –30.60%.
解释：
B is correct.
The total costs for the purchase is 300 × $30=$9000
Equity invested by Tim equals 0.5 × $9000=$4500
Tim borrowed amount is $9000-$4500=$4500
Interests are needed to paid by Tim at month end is $4500×0.04=$180
Dividends are received at month end is $1×300=$300
Proceeds on stock sale=$25×300=$7500
Total commissions paid by Tim is $5+$5=$10
Net gain/loss=-$9000-$180+$300+$7500-$10=$-1390
The initial investment of Tim is $4505
The rate of return on this investment for the one-month period is −$1390/$4505=-30.85%
在Tim‘s initial investment是4505，是因为需要加上五元钱的手续费，但是五元钱的手续费不是在倒数第三部算net gain的时候减去了吗？
老师这道题目题库里类似的一道题目：
Daisy shorts 1800 shares of KIY company's stock on a 30% initial margin and the broker's lending rate is 2.5%. The current price is $65 per share. The broker charges a commission of $0.02 per share on sales but purchases for free. KIY stock pays an annual dividend of $1 per share. One year later, Daisy buys the 1800 shares at a price of $58 per share and close the position. calculating the rate of return.
答案：1800x65=11700. 11700x0.3=35100. lending interest：2047.5，dividend：1800，commission：36.
return on investment: 1800x65 - 1800x58-2047.5 -1800=8716.5
Yield: 8716.5/(35100+36)=24.81%
想麻烦老师解释一下为什么Daisy题库里面这道题和本题的Net gain有什么不同的地方？为什么本题的最后Net gain 有加又有减，但是Daisy题的哪一步就是只有减，最后的commission36和35100加在一起？辛苦老师啦
