input: Write a python function to determine the letter grade based on a given percentage.
  assert grade(95) == 'A'
  assert grade(85) == 'B'
  assert grade(75) == 'C'
  assert grade(65) == 'D'
  assert grade(55) == 'F'
output: def grade(percentage):
      if percentage >= 90:
          return 'A'
      elif percentage >= 80:
          return 'B'
      elif percentage >= 70:
          return 'C'
      elif percentage >= 60:
          return 'D'
      else:
          return 'F'