## Solution to 1996 Problem 75

 The relevant equation is \begin{align*}\frac{\Delta Q}{\Delta t} = k A \frac{T_1 - T_2}{l}\end{align*}$\frac{\Delta Q}{\Delta t}$ is the rate at which heat is conducted through the substance, $A$ is the area of the substance, $l$ is the length of the substance, $T_1 - T_2$ is the difference in temperature between the two sides of the substance. We assume that the temperature difference and area are the same for both windows. We also assume that the thin layers of glass in Window B are thin enough so that heat is conducted across them instantaneously. The ratio of heat flow through window A to the ratio of heat flow through window B is then\begin{align*}\frac{k_A \cdot A \cdot \displaystyle \frac{T_1 - T_2}{l_A}}{k_B \cdot A \cdot \displaystyle \frac{T_1 - T_2}{l...Therefore, answer (D) is correct.