input: Write a function to calculate the average of a list of numbers.
  assert get_average([1, 2, 3, 4, 5]) == 3.0
  assert get_average([10, 20, 30, 40, 50]) == 30.0
  assert get_average([5, 10, 15]) == 10.0
output: def get_average(numbers):
      return sum(numbers) / len(numbers)