input: Write a python function to calculate the average of a list of numbers.
  assert average_of_list([3, 7, 9, 4, 5]) == 5.6
  assert average_of_list([10, 20, 30, 40, 50]) == 30.0
  assert average_of_list([1, 2, 3, 4, 5]) == 3.0
output: def average_of_list(numbers):
      return sum(numbers) / len(numbers)