A History Teacher Assigns Letter Grades on a Test According

A history teacher assigns letter grades on a test according to the following scheme:
A: 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: Bottom 10%
Scores on the test are normally distributed with a mean of 65 and a standard deviation of 13.9. Find the numerical limits for each letter grade.

A) A: Above 78
B: Between 68 and 78
C: Between 54 and 67
D: Between 50 and 53
F: Below 50

B) A: Above 83
B: Between 68 and 83
C: Between 53 and 67
D: Between 47 and 52
FBelow 47

C) A: Above 83
B: Between 72 and 83
C: Between 53 and 71
D: Between 47 and 52
F: Below 47

D) A: Above 78
B: Between 68 and 78
C : Between 54 and 67
D: Between 47 and 53
F: Below 47