Water flows in a streamline motion through a horizontal pipe of circular cross-section as shown in the figure. The pressure difference of water between P and Q is $15~Nm^{-2}.$ The area of cross-section at P and Q are 40 $cm^{2}$ and $20~cm^{2}$, respectively. The rate of flow of water through the pipe, in $cm^{3}s^{-1}$, is: [Take density of water $=1000~kg~m^{-3}]$ [Image Placeholder]