$4000+($60−$50)X200shares这里不明白，公式怎么推导的？

问题如下：

A trader has purchased 200 shares of a non-dividend-paying firm on margin at a price of $50 per share. The leverage ratio is 2.5. Six months later, the trader sells these shares at $60 per share. Ignoring the interest paid on the borrowed amount and the transaction costs, what was the return to the trader during the six-month period?

选项：

A.

20 percent

B.

33.33 percent

C.

50 percent

解释：

The return is 50 percent. If the position had been unleveraged, the return would be 20% = ($60 −$ 50)/ $50. Because of leverage, the return is 50% = 2.5 X 20%.

From another way to think about it: The minimum margin requirement is 40%=1/2.5, so he contribute $4000=40% X (200shares X $50) to the margin. Six month later, the amount in the margin is $6000= $4000+($60−$50)X200shares. Hence, the return is 50% = ($6000−$4000)/$4000