请教一个prep里面的关于标准方差的题

brandie_who

For a Ceitain Examination, a score of 58 was 2 standard deviation below the mean, and a score of 98 was 3 standard deviations above the mean. What was the mean score for the examination?

A. 74

B. 76

C. 78

D. 80

E. 82

answer: A

不太明白什么叫“was 2 standard deviation below the mean”...

platobungee

“was 2 standard deviation below the mean”... 就是比平均值低两个标准差

这题其实你不用知道标准差怎么来的你也能做出来啊，人家都说了是标准差了，所以一个标准差肯定是常量，这里你可以设为d，那根据题意有：

58 = Mean - 2d

98 = Mean + 3d

二元一次方程很容易就得Mean = 74了

brandie_who

多谢柏拉图的指教