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? Answer: 10.5

已经确认题目和答案都没有打错。

模考第一道数学就是这个，完全不知道问题是什么意思？思路又是什么？烦请过路的NN帮忙解释一下吧！真是万分感谢啦！

I would recommend a quick review of what standard deviation means...

1 standard deviation less than mean = 13.5 - 1.5 = 12
1 standard deviation more than mean = 13.5 + 1.5 = 15
2 standard deviation less than mean = 13.5 - 1.5*2 = 10.5
2 standard deviation more than mean = 13.5 + 1.5*2 = 16.5