A cylindrical steel rod of length $0.10 \mathrm{~m}$ and thermal conductivity $50 \mathrm{~W} \cdot \mathrm{m}^{-1} \mathrm{~K}^{-1}$ is welded end to end to copper rod of thermal conductivity $400 \mathrm{~W} \cdot \mathrm{m}^{-1} . \mathrm{K}^{-1}$ and of the same area of cross section but $0.20 \mathrm{~m}$ long. The free end of the steel rod is maintained at $100^{\circ} \mathrm{C}$ and that of the copper and at $0^{\circ} \mathrm{C}$. Assuming that the rods are perfectly insulated from the surrounding, the temperature at the junction of the two rods-