A set of final examination grades in a course is normally distributed with a mean of 73 and a standard deviation of 8.

  1. What is the probability of getting a grade below 91 on the exam?
  2. What is the probability that a student scored between 65 and 89?
  3. If the professor grades on a curve (gives A’s to the top 10% of the class, regardless of the score), are you better off with a grade of 81 on this exam or a grade of 68 on a different exam, where the mean is 62 and the standard deviation is 3? Explain why.
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    $\begingroup$ Welcome to the site. This looks like homework or an exam question. If it is, please add the self-study tag. $\endgroup$
    – Peter Flom
    Commented Oct 27, 2013 at 18:08
  • $\begingroup$ We welcome questions like this, @Megan Brown, but we treat them differently (see our help page). Please tell us what you understand & have tried already, & we'll provide some hints to help get you unstuck. $\endgroup$ Commented Oct 27, 2013 at 18:28
  • $\begingroup$ I am trying to calculate the z-score. Using the formula z= y-mean / standard deviation. Can't find our what value to use for y $\endgroup$
    – user31966
    Commented Oct 27, 2013 at 18:30

1 Answer 1


What you need to do is standardize those grades so you can use the standard Normal Distribution which is extensively tabulated. Try that first and should you have any problems let us know.


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