input: Write a function to calculate the average of a list of positive integers and return the result. If the list is empty, return 0.
  assert calculate_average([1, 5, 6, 7, 8])==5.4
  assert calculate_average([3, 5, 7, 9])==6.0
  assert calculate_average([10])==10.0
  assert calculate_average([])==0
output: def calculate_average(numbers):
      if not numbers:
          return 0
      return sum(numbers) / len(numbers)