The surface of water in a water tank of cross section area  $750{\text{ cm}}^2$ 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  $500{\text{ mm}}^2$  is  $60\text{ cm}/\text{s}$. At that instant, $\frac{\text{dh}}{\text{dt}}$ is  $x\times {10}^{-3}\text{ m}/\text{s}$. The value of x will be .