input: def calculate_average(numbers: List[float]) -> float:
      """ Calculate the average of a list of numbers.
      >>> calculate_average([1.0, 2.0, 3.0, 4.0])
      2.5
      >>> calculate_average([10.0, 20.0, 30.0])
      20.0
      >>> calculate_average([5.5, 6.5, 7.5])
      6.5
      """
output:     total = sum(numbers)
      count = len(numbers)
      return total / count