Two long rods of the same diameter, one made of brass (k = 85 W/m. K.) and the other made of copper (k = 375 W/m. K.) have one of their ends inserted into a furnace. Both rods are exposed to the same environment. At a section 10.5 cm away from the furnace end, the temperature of the brass rod is 120°C. At what distance from the furnace end, the same temperature would be reached in the copper rod? Derive the formula used.