For a temperature difference $\Delta T=20.0^{\circ} \mathrm{C},$ one slab of material conducts $10.0 \mathrm{W} / \mathrm{m}^{2} ;$ another of the same shape conducts $20.0 \mathrm{W} / \mathrm{m}^{2} .$ What is the rate of heat flow per square meter of surface area when the slabs are placed side by side with $\Delta T_{\mathrm{tot}}=20.0^{\circ} \mathrm{C} ?$