input: Write a python function to calculate the mean of a list of numbers.
  assert calculate_mean([1, 7, 11, 13]) == 8.0
  assert calculate_mean([2, 4, 6, 8, 10]) == 6.0
  assert calculate_mean([5, 15, 25, 35, 45]) == 25.0
output: def calculate_mean(numbers):
      return sum(numbers) / len(numbers)