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 68 and a standard deviation of 12.5.

find the numerical limits for each letter grade