input: Write a python function to calculate the mean of a list of numbers.
  assert calculateMean([4, 9, 3, 5]) == 5.25
  assert calculateMean([1, 2, 3, 4, 5, 6]) == 3.5
  assert calculateMean([10, 20, 30, 40]) == 25.0
output: def calculateMean(numbers):
      return sum(numbers) / len(numbers)