input: Write a function to calculate the average of a list of positive integers and output the result. 
  assert calculate_average([1, 5, 6, 7, 8])==5.4
  assert calculate_average([10, 20, 30, 40, 50])==30.0
  assert calculate_average([1, 3, 5, 7, 9])==5.0
  assert calculate_average([2, 4, 6, 8, 10])==6.0
  assert calculate_average([1, 2, 3, 4, 5])==3.0
output: def calculate_average(numbers):
      return sum(numbers) / len(numbers)