NO.PZ2020020601000017
问题如下：
A trader shorts 100 shares when the price is USD 50. The initial margin and maintenance margin are 150% and 125%. What is the initial margin required? How high can the share price go before further margin is required? (Ignore interest payments.)
选项：
解释：
The trader is initially required to contribute USD 2,500 in addition to the USD 5,000 obtained from selling the shares, creating a margin balance of USD 7,500. If the USD share price rises to X, the maintenance margin becomes 100 * 1.25X = 125X. This is greater than the 7,500 margin balance when X > 7,500/125, that is, when X > 60.
6250只是股价在50情况下的maintenance margin，目的是为了告诉你maintenance margin 是125% 为了后面的计算。 而initial margin才是你的margin balance，可以想成是你账户里的钱。但同时因为有100 shares 是需要还回去的。 题目问的是在不追缴保证金的前提下（用现有7500 margin balance）可以允许stock涨到多少钱（因为是short方）。所以用margin balance 7500 / 125% 算出最高允许stock价值（符合mantntenance margin的125%要求），再除以100算出每股价格60， 这就是在不被margin call前提下，股票最高能到的价格。
