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%
我理解不了为什么 1.-9000 ，怎么去理解？ 价值9000的股票卖出去了 所以是-9000？ 因为我始终觉得 我实际花了4500啊 为什么-9000... 2. Total commissions paid by Tim is $5+$5=$10 买和卖都要5$的comission,分子那么一大串是什么东西？？ 计算法则是什么？？？分母我能理解是实际支出的。。已经算了5刀的commission了，为什么分子还要算成10 不算成5.
