input: Write a python function to calculate the average of a list of grades.
  assert average_grade([90, 99, 75, 72, 85]) == 84.2
  assert average_grade([80, 90, 100, 70, 60]) == 80.0
  assert average_grade([100, 100, 100, 100, 100]) == 100.0
output: def average_grade(grades):
      return sum(grades) / len(grades)