input: 
  def calculate_mean(numbers):
      total_sum = 0
      for num in numbers:
          total_sum += num
      mean = total_sum / len(numbers)
      return mean
  
  # Example:
  # for calculate_mean([10, 20, 30]) == 20.0
  
output:     total_sum = 0
      for num in numbers:
          total_sum += num
      mean = total_sum / len(numbers)
      return mean