请教OG-PS-416

416. If a certain sample of data has a mean of 20.0 and a standard deviation of 3.0, which of the following values is more than 2.5 standard deviation from the mean?

(A) 12.0 (B) 13.5 (C) 17.0 (D) 23.5 (E) 26.5

OG解释说 to be more than 2.5 standard deviations from the mean of 20, the value must be either greater than 20+2.5(3)=27.5, or less than 20-2.5(3)=12.5. The best answer is A.

我对这道题根本是不知道如何下手!!! 2.5(3)指的是什么意思??

我做的时候, 是仅从题意及对standard deviation的概念的大致了解, 知道这题答案一定是与20.0相差最大的那个,

但毕竟这样选法总是心中难安哪.请哪位高手指点一下! 不胜感激!!