The design of a physical instrument requires that there be a constant difference in length of 10 cm between an iron rod and a copper cylinder laid side by side at all temperatures. If αբₑ = 11 x 10⁻⁶ °C⁻¹ and α꜀ᵤ = 17 x 10⁻⁶ °C⁻¹ , their lengths are