Two cylindrical rods have the same mass. One is made of silver (density = 10 500 kg/m3), and one is made of iron (density = 7860 kg/m3). Both rods conduct the same amount of heat per second when the same temperature difference is maintained across their ends. What is the ratio (silver-to-iron) of
(a) the lengths and
(b) the radii of these rods? Give your answers as numbers with no units.