input: Write a python function to calculate the average of a list of numbers.
  assert calculate_average([3, 4, 5]) == 4.0
  assert calculate_average([1, 2, 3, 4, 5]) == 3.0
  assert calculate_average([10, 20, 30]) == 20.0
output: def calculate_average(numbers):
      return sum(numbers) / len(numbers)