GWD 18-18 MATH

missouyang

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%

答案是D偶不会的说.哪位好心人给讲讲吧

casualwalk

这类似正态分布吧，

实际上题目中给出P{(X-m)的绝对值<d}=0.68

所以P{(X-m)的绝对值>=d}=1-0.68=0.32；

因为是均衡分布，所以

P{(X-m)>=d}＝0.5* P{(X-m)的绝对值>=d} =0.16；

要求的percent of the distribution is less than m + d

即P{X<m+d}=P{(X-m)<d}=1-P{(X-m)>=d}=0.84

missouyang

many  thanks !!!!!