​Heat loss, $h$, per hour through a glass window varies directly with the difference between the indoor and outdoor temperature, $d$, and inversely with the thickness of the glass, $g$.  

1. Write an expression that will find the heat loss based on the difference of the temperature and the thickness of the glass. Do not forget the constant of proportionality!

   Answer (a):

   $h=\frac{kd}{g}$

2. The Johnson family has a window that is $0.3$ cm thick that loses $2.4$ BTU of energy each hour when the outside temperature is $50º\text{F}$ and the inside temperature is $70º\text{F}$. Use this information to determine the constant of proportionality.

   Step (b):

   $2.4=\frac{k(70-50)}{0.3}$

3. The Johnson’s decide to replace the window with one that is $1.5$ cm thick. How much energy will be lost if the inside temperature is $70º\text{F}$ and the outside temperature is $30º\text{F}$?

   Hint (c):

   Use your formula from part (a).