[求助]一道关于standard deviation的题目，求解题思路~

一道出现在prep里面的数学题，做题的时候不是很明白，虽然蒙对了，但是想请教各位牛人，正确的解题思路是怎样的呀？

题目是这样的：

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, 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?

选项分别是1，2，3，4和5，正确答案是2.

THX~!!