input: Write a function to find the average of numbers in a list.
  assert find_average([3, 5, 7, 9, 11]) == 7.0
  assert find_average([1, 2, 3, 4, 5, 6]) == 3.5
  assert find_average([10, 20, 30, 40, 50]) == 30.0
output: def find_average(numbers):
      return sum(numbers) / len(numbers)