input: Write a python function to calculate the average of a list.
  assert average_list([4, 2, 6, 8, 10]) == 6.0
  assert average_list([1, 2, 3, 4, 5]) == 3.0
  assert average_list([10, 20, 30, 40, 50]) == 30.0
output: def average_list(numbers):
      return sum(numbers) / len(numbers)