请教一道读了N遍也没读懂的题

 70，75，80，85，90，105，105，130，130，130

This 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, how many of the 10 running times are more than 1 standard deviation below the mean of the 10 running times?

看到别人的帖子突然想到这道题怎么做，希望对同样不知道这道题怎么做的人有点帮助。

平均值是100，100-22.4*1=77.6 ,这样就只有70,75 两个数满足题目条件,