The letter grades on the midterm exam given in a large managerial statistics class are normally distributed with mean 75 and standard deviation 9. The instructor of this class wants to assign an A grade to the top 10% of the scores, a B grade to the next 10% of the scores, a C grade to the next 10% of the scores, a D grade to the next 10% of the scores and an F grade to all scores below the 60th percentile of this distribution. For each possible letter grade, find the lowest acceptable score.