A three-dimensional velocity field is given by $V = 5x^2y\,\mathbf{i} + Cy\,\mathbf{j} - 10xyz\,\mathbf{k}$, where $\mathbf{i},\mathbf{j},\mathbf{k}$ are unit vectors in $x,y,z$ directions. If $V$ describes an incompressible fluid flow, the value of $C$ is