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