A copper rod and an aluminum rod of equal diameter are joined end to end in good thermal contact.

A copper rod and an aluminum rod of equal diameter are joined end to end in good thermal contact. The temperature of the free end of the copper rod is held constant at 100°C, and that of the far end of the aluminum rod is held at 0°C. If the copper rod is 0.30 m long, what must be the length of the aluminum rod so that the temperature at the junction is 50°C?