input: Write a function to calculate the average of a list of positive integers and output the result.
  
  `
  assert calculate_average([1, 5, 6, 7, 8]) == 5.4
  assert calculate_average([10, 20, 30, 40, 50]) == 30.0
  assert calculate_average([1]) == 1.0
  assert calculate_average([]) == None
  assert calculate_average([2, 4, 6, 8]) == 5.0
  `
output: def calculate_average(numbers):
      if not numbers:
          return None
      return sum(numbers) / len(numbers)