Water flows in a horizontal tube (see figure). The pressure of water changes by $700 \mathrm{Nm}^{-2}$ between A and $B$ where the area of cross section are $40 \mathrm{~cm}^2$ and $20 \mathrm{~cm}^2$, respectively. Find the rate of flow of water through the tube. $\left(\right.$ density of water $\left.=1000 \mathrm{kgm}^{-3}\right)$