input: Write a python function to calculate the mean of a list of numbers.
  assert calculate_mean([2, 6, 9, 8, 4, 5]) == 5.8
  assert calculate_mean([1, 2, 3, 4, 5, 6]) == 3.5
  assert calculate_mean([10, 20, 30, 40, 50]) == 30.0
output: def calculate_mean(numbers):
      return sum(numbers) / len(numbers)