A humanities professor assigns letter grades on a test according to the following scheme. A: Top 11% of scores B: Scores below the top 11% and above the bottom 64% C: Scores below the top 36% and above the bottom 22% D: Scores below the top 78% and above the bottom 9% F: Bottom 9% of scores Scores on the test are normally distributed with a mean of 70.6 and a standard deviation of 9.3. Find the minimum score required for an A grade. Round your answer to the nearest whole number, if necessary.

The minimum score required for an A grade is 82.

Step-by-step explanation:

Problems of normally distributed samples are solved using the z-score formula.

In a set with mean and standard deviation , the zscore of a measure X is given by:

The Z-score measures how many standard deviations the measure is from the mean. After finding the Z-score, we look at the z-score table and find the p-value associated with this z-score. This p-value is the probability that the value of the measure is smaller than X, that is, the percentile of X. Subtracting 1 by the pvalue, we get the probability that the value of the measure is greater than X.

In this problem, we have that:

Find the minimum score required for an A grade.

Top 11%, which is at least 100-11 = 89th percentile. So the value of X when Z has a pvalue of 0.89. So X when Z = 1.225.

The minimum score required for an A grade is 82.