prep的一条数学求教~~~

swuliwuli

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 running times?

70, 75,80, 85,90,105,105,130,130,130

方差为1 怎么做啊 脑子里一团糊啊

firstfengrui

我是这样想的哈
平均数是100。
又因为比方差大22.4+1
所以只有70.75.答案为2
不知对否