input: Write a python function to calculate the mean of a list of numbers.
  assert calculate_mean([0.07, 0.69, 0.29, 0.17, 0.11, 0.51, 0.5]) == 0.3542857142857143
  assert calculate_mean([1, 2, 3, 4, 5]) == 3.0
  assert calculate_mean([10, 20, 30]) == 20.0
output: def calculate_mean(numbers):
      return sum(numbers) / len(numbers)