一道PREP数学的统计题

sparrow_zhu

问一个PREP的数学题，统计方面的，基本忘记了。题目是：

The arithmetic mean and standard deviation of a certain normal distribution are 13.5 and 1.5, respectively, what value is exactly 2 standard deviations less than the mean?

为什么答案是10.5？

mymengming

13.5-1.5×2