The surface of water in a water tank of cross section 750a 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 $$500 \mathrm{~mm}^{2}$$ 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 __________.