input: Write a python function to calculate the average of a list of numbers.
  assert avg([1, 2, 3, 4, 5]) == 3.0
  assert avg([10, 20, 30, 40, 50]) == 30.0
  assert avg([5, 15, 25, 35, 45]) == 25.0
output: def avg(numbers):
      return sum(numbers) / len(numbers)