PREP一道数学关于标准差 在线等

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 students to run a distance of 400 meters. If the std. deviation of the 10 running times is 22.4 sec, rounded to nearest tenth of a second , how many of the 10 running times are more than 1 Std deviation below the mean of the 10 running times?

1) One
2) two
3)three
4) four
5)five

Answer is : 2

 
这个题目怎么回事 我不太懂 请牛人解释下 这个more than 1 standard deviation 怎么理解