Aphilosophy professor assigns letter grades on a test according to the following scheme. a: top 12% of scores b: scores below the top 12% and above the bottom 57% c: scores below the top 43% and above the bottom 17% d: scores below the top 83% and above the bottom 9% f: bottom 9% of scores scores on the test are normally distributed with a mean of 81.2 and a standard deviation of 8.1