请教一道数学题

3smoviesalon

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 school children to run a distance of 400 meters. If the standard deviation of the 10 running time 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

ANS: B

这道题不是很理解最后一句话，也不知道怎么解？ 谢谢！

britney-gao

The sum of 10 running times is 1000. Therefore, the mean (arithmetic average) is 100.
Therefore, 1 standard deviation (22.4) below the mean is 77.6

Below 77.6, we have two : 70 and 75.

longger

 啊啊啊啊。。。　千万小心啊。。。　
辛辛苦苦算完了，没看清below这个词。。。以为是　x < ７７。６ or x > 122.4, 结果选了５。。。

数学题真是不能马虎啊。。。　千万千万。。。　

anonym1106

GMAC就是喜欢玩阴的!!

wangjianghua

是啊&nbsp; 很多题都错在没看清题意&nbsp;&nbsp; 所以做题时一定要小心.