input: def calculate_average(nums: List[float]) -> float:
      """ Calculate the average of a list of five numbers.
      >>> calculate_average([10.0, 20.0, 30.0, 40.0, 50.0])
      30.0
      >>> calculate_average([5.0, 10.0, 15.0, 20.0, 25.0])
      15.0
      """
output:     if len(nums) != 5:
          raise ValueError("The list must contain exactly five numbers.")
      return sum(nums) / 5