Three rods of identical cross-section and lengths are made of three different materials of thermal conductivity $K_1,K_{2}and{K}_3,$ respectively. They are joined together at their ends to make a long rod (see figure). One end of the long rod is maintained at $100 {}^{o}C$ and the other at $0 {}^{o}C$ (see figure). If the joints of the rod are at $70 {}^{o}C$ and $20 {}^{o}C$ in steady state and there is no loss of energy from the surface of the rod, the correct relationship between $K_1,K_{2}and{K}_3$ is :