Final averages are typically approximately normally distributed with a mean of 72 and a standard deviation of 12.5. Your professor says that the top 8% of the class will receive an A, the next 20% a B, the next 42% a C, the next 18% a D, and the bottom 12% an F.

Question

Final averages are typically approximately normally distributed with a mean of 72 and a standard deviation of 12.5. Your professor says that the top 8% of the class will receive an A, the next 20% a B, the next 42% a C, the next 18% a D, and the bottom 12% an F.
a. What average must you exceed to obtain an A?
b. What average must you exceed to receive a grade better than a C?
(By the way, we won’t follow this rule in this class, we’ll follow the syllabus instead).

PsyDAG · Answer

What rule?

Find table in the back of your statistics text labeled something like "areas under normal distribution" to find the proportion/probability (.08A, .08+.20>C) and related Z scores. Insert in equation below to find raw score.

Z = (score-mean)/SD