A copper rod and a steel rod of the same length and area are joined end to end. The free end of the copper is kept at $100\ ^\circ\text{C}$ and that of the steel at $0\ ^\circ\text{C}$. What is the temperature of the joint? ($k$: copper $385\ \text{W m}^{-1}\text{K}^{-1}$, steel $50\ \text{W m}^{-1}\text{K}^{-1}$.)