最近发现官网的sample没了，在版上找到了一个sample，关于margin的题都没有删掉。关键大纲上还写了margin。所以我也有点糊涂了，请问大家知道现在还考关于margin的计算题么？类似这种：
Eric and Jason each sell a different stock short at the beginning of the year for a price of
800. The margin requirement for each investor is 50% and each will earn an annual
effective interest rate of 8% on his margin account.
Each stock pays a dividend of 16 at the end of the year. Immediately thereafter, Eric
buys back his stock at a price of (800 - 2X), and Jason buys back his stock at a price of
(800 + X).
Eric’s annual effective yield, i, on the short sale is twice Jason’s annual effective yield.
Calculate i.

这题给的答案里面，for Eric， (Value of contributions) 400 (1+i) + 800 – 2X + 16，不是很懂为什么会有个400（1+i）在里面……到处找资料也没看到有解释的。
多谢回答。