input: Write a python function to determine a student's grade based on their marks in math, history, and english.
  assert determine_grade({'math': 95, 'history': 85, 'english': 90}) == 'A'
  assert determine_grade({'math': 80, 'history': 75, 'english': 85}) == 'B'
  assert determine_grade({'math': 70, 'history': 72, 'english': 68}) == 'C'
  assert determine_grade({'math': 60, 'history': 65, 'english': 55}) == 'D'
  assert determine_grade({'math': 50, 'history': 55, 'english': 45}) == 'F'
output: def determine_grade(marks):
      average = sum(marks.values()) / len(marks)
      if average >= 90:
          return 'A'
      elif average >= 80:
          return 'B'
      elif average >= 70:
          return 'C'
      elif average >= 60:
          return 'D'
      else:
          return 'F'