input: Write a function to calculate the average of a list of positive integers and output the result. If the list is empty, return 0.
  assert calculate_average([1, 5, 6, 7, 8])==5.4
  assert calculate_average([2, 4, 6, 8, 10])==6.0
  assert calculate_average([3, 6, 9, 12])==7.5
  assert calculate_average([15, 20, 25, 30])==22.5
  assert calculate_average([])==0
output: def calculate_average(lst):
      if not lst:
          return 0
      return sum(lst) / len(lst)