问一道PP上看不懂的题！请指教！

bianyuan021

比平均数少2倍标准差的数是多少，应该是13.5-1.5*2=10.5

kepei7763

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

10.5

11.0

11.5

12.0

12.5

这个题是什么意思？怎么做呢？