input: Write a python function to calculate the average of a list of numbers.
  assert calculate_average([1, 7, 8, 4, 9]) == 5.6
  assert calculate_average([10, 20, 30, 40]) == 25.0
  assert calculate_average([2, 4, 6, 8, 10]) == 6.0
output: def calculate_average(numbers):
      return sum(numbers) / len(numbers)