难题请教：高手请进！

JohnDong

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?

70  ;   75   ;   80    ;    85;    90    ;   105   ;    105   ;   130    ;     130    ;      130        

choices:

one

two

three

four

five

[此贴子已经被作者于2009/10/28 6:16:46编辑过]

icewolf324

这题答案是2吗？

JohnDong

是的，不过不太明白问题的意思？请教，二楼你是如何理解的。

bearwinnie

1. 算出mean=100

2. 因为sd=22.4，所以below the mean more than 1sd的应该小于100-22.4=77.6,

3.那么给出的数字里只有前两个是小于77.6. So...

JohnDong

呵呵，so，1sd就是1个标准差就是22.4，就是在平均值的基础上减掉22.4，当时是没太理解1个标准差，懂了，谢谢！