[求助]一道标准方差的题.

羽依依

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

An: 74.

谢谢

lewis

58=M-2D

98=M+3D

hjgjfgfb

非常感谢，我一直不知道这个几个standard deviations是什么意思...