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 $30 \mathrm{~cm} / \mathrm{s}$. At that instant, $\frac{d h}{d t}$ is $x \times 10^{-3} \mathrm{~m} / \mathrm{s}$. The value of $x$ will be ____________.