input: Write a function to calculate the average of a list of numbers.
  assert average([1, 2, 3, 4, 5, 6, 7]) == 4
  assert average([10, 20, 30, 40, 50]) == 30
  assert average([5, 10, 15]) == 10
output: def average(numbers):
      return sum(numbers) / len(numbers)