Q18:

A certain characteristic in a large population has a distribution that is symmetric about the mean m.  If 68 percent of the distribution lies within one standard deviation d of the mean, what percent of the distribution is less than m + d ?

A.     16%

B.     32%

C.     48%

D.     84%

E.      92%

请指教这个题目怎么解?

请楼上讲一下这个等式是怎么建立的．．．不是太明白这个知识点．．麻烦！