Question: A history professor assigns letter grades according the following scale. Scores are normally distributed with a mean of 66 and a standard deviation of 12.8. Find the numerical limits for each letter grade. Round your answers to the nearest appropriate whole number.
A: scores in the top 10%
B: scores below the top 10% and above the bottom 60%
C: scores below the top 40% and above the bottom 20%
D: scores below the top 80% and above the bottom 10%
F: scores in the bottom 10%