请教：在KFC里少见的PREP题，题目都没看懂!

WillNmind

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?

10.5/11.0/11.5/12.0/12.5     Ans:A

WillNmind

多谢！似乎没什么数学含义啊？