input: Write a python function to return the largest number in a list.
  assert largest_num([50, 30, 65, 80]) == 80
  assert largest_num([1, 2, 3, 4, 5]) == 5
  assert largest_num([-10, -20, -3, -40]) == -3
output: def largest_num(numbers):
      return max(numbers)