input: Write a python function to calculate the average of a list of positive integers.
  assert calculate_average([1, 5, 6, 7, 8]) == 5.4
  assert calculate_average([10, 20, 30, 40, 50]) == 30.0
  assert calculate_average([2, 4, 6, 8, 10]) == 6.0
output: def calculate_average(numbers):
      return sum(numbers) / len(numbers)