一道Prep数学,没明白什么意思,请大家帮忙看看吧,多谢了

9.4  9.9  9.9  9.9  10.0  10.2  10.2  10.5
The mean and the standard deviation of the 8 numbers shown are 10.0 and 0.3, respectively.  What percent of the 8 numbers are within 1 standard deviation of the mean?

  90%
  85%
  80%
  75%
  70%