请教一道PREP题

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 tunning times?

没有搞明白问题是什么意思，请教各位指点一下。选择答案有1，2，3，4，5

好的,谢谢!