input: Write a python function to print numbers from 15 to 10 using a while loop.
  assert print_numbers() == "15\n14\n13\n12\n11\n10\n"
output: def print_numbers():
      result = ""
      num = 15
      while num >= 10:
          result += str(num) + "\n"
          num -= 1
      return result