input: Write a function to calculate the average of a list of 10 numbers.
  assert avg_of_10_numbers([1,2,3,4,5,6,7,8,9,10]) == 5.5
  assert avg_of_10_numbers([10,20,30,40,50,60,70,80,90,100]) == 55.0
  assert avg_of_10_numbers([5,5,5,5,5,5,5,5,5,5]) == 5.0
output: def avg_of_10_numbers(numbers):
      if len(numbers) != 10:
          raise ValueError("The list must contain exactly 10 numbers.")
      return sum(numbers) / 10