[求助]

叮叮当当

求助一道数学题: 一列数,平均数为20, 方差为3.0, 问: which one of the following values is more than 2.5 standard deviations from the mean .

a.12, b.13.5, c.23.5, d.26.5 答案给的是a.

请教高人指点解题方法