input: Write a function to calculate the average of a list of positive integers and return the result as a float. If the list is empty, return 0.0.
  
  assert calculate_average([1, 5, 6, 7, 8]) == 5.4
  assert calculate_average([10, 20, 30]) == 20.0
  assert calculate_average([]) == 0.0
output: def calculate_average(numbers):
      if not numbers:
          return 0.0
      return sum(numbers) / len(numbers)