A block of mass 30 kg and an empty and light Box of equal volume are attached by an ideal spring of force constant k = 400N/m. the whole system is floating in water of density $\left({\rho }_w\right)={10}^{3}kg/m^3$. The upper block is half submerged into water. Take the value of acceleration due to gravity g = 10$m/s^2$ . If the compression in spring is 10k cm. Then find the value of k.