input: Write a function to calculate the average of five numbers entered by a user. If the input numbers are less than five, return None. 
  assert calculate_average([4, 5, 6, 7, 8])==6.0
  assert calculate_average([1, 2, 3])==None
  assert calculate_average([])==None
  assert calculate_average([10, 20, 30, 40, 50])==30.0
output: def calculate_average(numbers):
      if len(numbers) < 5:
          return None
      return sum(numbers) / 5