Mysql how long would it take to add an index to 80 million rows

7. test average and grade write a program that asks the user to enter five test scores. the program should display a letter grade for each score and the average test score. write the following methods in the program: calcaverage: this method should accept five test scores as arguments and return the average of the scores. determinegrade: this method should accept a test score as an argument and return a letter grade for the score, based on the following grading scale: score letter grade 90-100 a 80-89 b 70-79 c 60-69 d below 60 f