new_contents: import pandas as pd
  
  grades_df = pd.DataFrame(
      data={'exam1': [43, 81, 78, 75, 89, 70, 91, 65, 98, 87],
            'exam2': [24, 63, 56, 56, 67, 51, 79, 46, 72, 60]},
      index=['Andre', 'Barry', 'Chris', 'Dan', 'Emilio',
             'Fred', 'Greta', 'Humbert', 'Ivan', 'James']
  )
  
  
  def convert_one(grade):
      if grade >= 90:
          return 'A'
      elif grade >= 80:
          return 'B'
      elif grade >= 70:
          return 'C'
      elif grade >= 60:
          return 'D'
      else:
          return 'F'
  
  
  def convert_grades(grades):
      '''
      Returns a new DataFrame of converted
      numerical to letter grades.
      '''
  
      return grades.applymap(convert_one)
  
  new_df = convert_grades(grades_df)
  print new_df
  
subject: Create convert_cols.py