In an experiment similar to Jean Baptiste Perrin's 1890 experiment, small identical particles having a density slightly greater than that of water are suspended in a test tube. At a height of 1 above the bottom of the tube, there are 96 particles per unit volume. At a height of 21 above the bottom of the tube, there are 72 particles per unit volume. How many particles per unit volume would be at a height of 31 above the bottom of the tube