input: Write a python function to calculate the average of a given list.
  assert calculate_average([2, 7, 5, 4]) == 4.5
  assert calculate_average([10, 20, 30, 40, 50]) == 30.0
  assert calculate_average([1, 1, 1, 1, 1]) == 1.0
output: def calculate_average(lst):
      return sum(lst) / len(lst)