Answer
$82^{\circ}F$
Work Step by Step
We first must find the value of H for the walls and for the glass:
$H_{walls} = \frac{A_{walls}\Delta T}{R_{walls}}= \frac{(300) \Delta T}{30}=10 \Delta T$
$H_{glass} = \frac{A_{glass}\Delta T}{R_{glass}}= \frac{(250) \Delta T}{1.8}=139\Delta T$
We divide the total heat loss by the total heat gain to find:
$\Delta T = \frac{1 \times 10^4}{149}=67^{\circ}F$
Thus, when it is 15 degrees outside, it is $82^{\circ}F$ inside.
