standard deviation

baller12q wrote:For a certain examination, a score of 58 was 2 standard deviations below the mean, and a score of 98 was 3 standard deviations above the mean. What was the mean score for the examination?

i dont understand standard deviation so can someone give me the basic info so that i can atleast get the simple questions right. please!

Aside: If the mean of a population is 10, the standard deviation is 2, then 12 would be one unit of standard deviation above the mean, 14 would be two units of standard deviation above the mean, 16 would be three units of standard deviation above the mean and so on.
Similarly, 8 would be one unit of standard deviation below the mean, 6 would be two unit of standard deviation below the mean, and so on.

For this question, let M=mean and let D=standard deviation. We get two equations:
(1) M - 2D = 58
(2) M + 3D = 98
We can solve this system of equations for M to get M=74