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