So, we have the total volume is s, and then the total mass of fluid that will enter through this will be minus s rho q bar n hat d s, that gives a total mass of fluid that will enter through the fluid. Now, if you look at the dimension let us say that the elementary volume of the fluid, the elementary volume within the surface element s element d s. If it is a d tau, then what will happen to the total mass of the fluid, and we have the total mass of fluid because our total volume is tau which will give us integral about tau rho d tau.