The document calculates the difference in pressure between two points in a pipe with different diameters but constant flow rate, assuming no energy loss. It shows that with a diameter of 0.15m the pressure is 3.14m and with a diameter of 0.3m the pressure is 0.2m, giving a pressure difference of 2.94m. It then repeats the calculation with the diameters reversed, giving a pressure difference of -2.94m.