## Precalculus (6th Edition) Blitzer

$1540\ Btu/hr$
Step 1. Based on the given conditions, we can set up the model equation as $R=\frac{k}{d}$ where $d$ is the thickness and $k$ is a constant. Step 2. At $d=3.5\ in$; we have $R=\frac{k}{3.5}=2200\ Btu/hr$. Thus $k=7700\ Btu\cdot in/hr$ Step 3. At $d=5\ in$, we have $R=\frac{7700}{5}=1540\ Btu/hr$