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 deviations from the mean?"若某抽样数据是２０的平均数且其标准偏差为３.0,下列那一个是在上述平均值中大于标准偏差2.5的值?"
A.12.0(A)
B.13.5
C.23.5
D.26.5
我问题是翻译不准,书后的英语解析特别是如何得出的A(12.0)看不懂,请年轻的NN们细心指导? | 
