The surface of water in a water tank of cross section area $750 {\mathrm{cm}}^2$ on the top of a house is $h \mathrm{m}$. above the tap level. The speed of water coming out through the tap of cross section area $500 {\mathrm{mm}}^2$ is $30 \mathrm{cm} {\mathrm{s}}^{-1}$. At that instant, $\frac{\mathrm{d}h}{\mathrm{d}t}$ is $x\times {10}^{-3} \mathrm{m} {\mathrm{s}}^{-1}$. The value of $x$ will be ______.