请教Prep上的2道数学

gchgeng

一、The lifetimes of all batteries produced by certain company in a year have a distribution that is symmetric about the mean m. If the distribution has a standard deviation of d, what percent of the distribution is greater than m+d?
(1) 68 percent of the distribution lies in the interval from m-d to m+d, inclusive
(2)16 percent of the distribution is less than m-d
答案是D，题干中给出的标准差这个条件怎么用呀？
 二、9.4 9.9 9.9 9.9 10 10.2 10.2 10.5 The mean and the standard deviation of the 8numbers shown are 10 and 0.3, respectively. What percent of the 8numbers are within 1standard deviation of the mean?  75%
给出的标准差的条件是什么意思？谢谢

sunnyfangyuan

题目二是问有在10-0.3~10+0,3之间的数占总数的百分比。就是9.7~10.3之间嘛，可以看出有6个数在这个区间，所以比值是6/8=75%

sunnyfangyuan

第一道就是画个数轴~，具体也不知道怎么细说·

shixiaoliu1986

题目1就是说对称分布的distribution，1告诉68%在m-d m+d之间，那么根据对称剩下的32%有一半比m+d大，即16%，同理第二个选项