原题是 : Suppose that the distribution of marks on an exam is closely
described by a normal curve with a mean of 65. The 84th percentile of this distribution is 75.
(a) What is the 16th percentile?
(b) What is the approximate value of the standard deviation of exam
marks?
(c) What z score is associated with an exam mark of 50?
(d) What percentile corresponds to an exam mark of 85?
(e) Do you think there were many marks below 35? Explain.
估计标准方差
可能会用到偰比谢夫定理和 Empirical Rule 可能就是那个什么中心定理的