input: Write a python function to find the mean of a list of numbers.
  assert find_mean([2, 4, 6, 8, 10]) == 6
  assert find_mean([1, 3, 5, 7, 9]) == 5
  assert find_mean([10, 20, 30, 40, 50]) == 30
output: def find_mean(numbers):
      return sum(numbers) / len(numbers)