input: Write a function to calculate the average of a list of positive integers and output the result. The function should return the average value. If the list is empty, the function should return None.
  
  def calculate_average(list_of_positive_integers):
      if not list_of_positive_integers:
          return None
      total = sum(list_of_positive_integers)
      avg = total / len(list_of_positive_integers)
      return avg
  
  assert calculate_average([1, 5, 6, 7, 8]) == 5.4
  assert calculate_average([10, 20, 30, 40, 50]) == 30.0
  assert calculate_average([]) == None
  assert calculate_average([1]) == 1.0
  assert calculate_average([100, 200, 300]) == 200.0
output: def calculate_average(list_of_positive_integers):
      if not list_of_positive_integers:
          return None
      total = sum(list_of_positive_integers)
      avg = total / len(list_of_positive_integers)
      return avg