input: def mean_median(arr: List[float]) -> (float, float):
      """Given an array of numbers, calculate and return the mean and median.
      >>> mean_median([10, 15, 20, 25, 30, 35])
      (22.5, 22.5)
      >>> mean_median([7, 8, 9, 10, 11, 12, 13])
      (10.0, 10.0)
      """
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)