Three rods of identical cross-section and lengths are made of three different materials of thermal conductivity $\mathrm{K}_1, \mathrm{~K}_2$ and $\mathrm{K}_3$, respectively. They are joined together at their ends to make a long rod (see figure). One end of the long rod is maintained at $100^{\circ} \mathrm{C}$ and the other at $0^{\circ} \mathrm{C}$ (see figure). If the joints of the rod are at $70^{\circ} \mathrm{C}$ and $20^{\circ} \mathrm{C}$ in steady state and there is no loss of energy from the surface of the rod, the correct relationship between $\mathrm{K}_1, \mathrm{~K}_2$ and $\mathrm{K}_3$ is