an english professor assigns letter grades on a test according to the following scheme. a: top 13 % of scores b: scores below the top 13 % and above the bottom 61 % c: scores below the top 39 % and above the bottom 22 % d: scores below the top 78 % and above the bottom 6 % f: bottom 6 % of scores scores on the test are normally distributed with a mean of 75.1 and a standard deviation of 9.1. find the numerical limits for a d grade. round your answers to the nearest whole number, if necessary.