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, 6, 9, 12]) == 7.5
  assert calculate_average([5]) == 5.0
  assert calculate_average([]) == 0
output: def calculate_average(lst):
      if not lst:
          return 0
      return sum(lst) / len(lst)