In a steady incompressible flow, the velocity distribution is given by $\bar{V}=3x\hat{i}-Py\hat{j}+5z\hat{k}$, where, $V$ is in $m/s$ and $x,y$ and $z$ are in $m$. In order to satisfy the mass conservation, the value of the constant $P$ (in $s^{-1}$) is _____________