A copper rod 0.60 m long and a steel rod 0.30 m long, each with a cross-section of 3.0 cm², are…

Step 1. Resistances. Copper: $\dfrac{0.60\ \text{m}}{385\ \text{W m}^{-1}\text{K}^{-1} \times 3.0 \times 10^{-4}\ \text{m}^2} = 5.19\ \text{K/W}$. Steel: $\dfrac{0.30\ \text{m}}{50\ \text{W m}^{-1}\text{K}^{-1} \times 3.0 \times 10^{-4}\ \text{m}^2} = 20.0\ \text{K/W}$.

Step 2. Heat flow. $\dfrac{100\ \text{K}}{25.2\ \text{K/W}} = 3.97\ \text{W}$.

Step 3. Joint. $100\ ^\circ\text{C} - 3.97\ \text{W} \times 5.19\ \text{K/W} = 79.4\ ^\circ\text{C}$.