The grade scale identifies a grade, which may be a symbol, along with an associated percent value. The percent, by definition, is a numerical value. The numerical values assigned to our letter grades are as follows: the letter “O” has a value of 95%, the letter “S+” has a value of 85%, the letter “S” has a value of 75%, the letter “S-” has a value of 65%, and the letter “N” has a value of 55%. In most cases, courses with letter grades are not included in GPA.