A non-isotropic solid metal cube has coefficients of linear expansion as: $5 \times 10^{-5} /{ }^{\circ} \mathrm{C}$ along the x-axis and $5 \times 10^{-6} /{ }^{\circ} \mathrm{C}$ along the y and the z -axis. If the coefficent of volume expansion of the solid is $\mathrm{C} \times 10^{-6} /{ }^{\circ} \mathrm{C}$ then the value of C is $\_\_\_\_$ .