You have already created a factor vector, grades_f, which assigns each numeric grade into one of five categories ('F', 'D', 'C', 'B', 'A'). Your goal is to:

1. Call the summary() function on your grades_f variable to output the number of occurrences of each factor grade.
2. Divide those counts by the total number of grades, computed by calling length() function on grades_f variable, to obtain the relative frequency of each grade.