input: 
  def calculate_mean_median(arr):
      """
      Calculate the mean and median of a given array.
      >>> calculate_mean_median([10, 15, 20, 25, 30, 35])
      (22.5, 22.5)
      """
output:     arr.sort()
      mean = sum(arr) / len(arr)
      if len(arr) % 2 == 0:
          median = (arr[len(arr)//2 - 1] + arr[len(arr)//2]) / 2
      else:
          median = arr[len(arr)//2]
      return (mean, median)