求助PREP 166 谢谢

sandraliuyi

166.
                17879-!-item-!-187;#058&011634

70, 75, 80, 85, 90, 105, 105, 130, 130, 130

The list shown consists of the times, in seconds, that it took each of 10 schoolchildren to run a distance of 400 meters.  If the standard deviation of the 10 running times is 22.4 seconds, rounded to the nearest tenth of a second, how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running times?

 

(A) One

(B) Two

(C) Three

(D) Four

(E) Five

答案选B。题目有点不大明白。谢谢各位了

shiverson

就是说，一个标准差是22.4，然后这十个人的mean是100sec，问这十个人有几个跑得在mean一下的时间，100－22.4，也就是70和75那两个人在这之外，B