SOLUTION: Scores on a certain exam are normally distributed with a mean of 90 and a variance of 25. What is the z-value for a score of 80?

Question 1151477: Scores on a certain exam are normally distributed with a mean of 90 and a variance of 25. What is the z-value for a score of 80?

Answer by rothauserc(4718) (Show Source): You can put this solution on YOUR website!
z-score formula is defined as
:
z-score = (X - mean)/standard deviation
:
standard deviation = square root of variance, for this problem
:
standard deviation = square root(25) = 5
:
z-score = (80 - 90)/5 = -2
:
Note to find the probability associated with this z-score, you lookup the z-score in the table of z-scores
:
If your table of z-scores is only for positive values, then lookup the probability associated with, in this case, 2. Then subtract that value from 1 to get the probability associated with -2
:
Why do you think this works?
:
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