In a house the temperature at the surface of a window is 25 ¡ãC. The temperature outside at the window surface is 5.0 ¡ãC. Heat is lost through the window via conduction, and the heat lost per second has a certain value. The temperature outside begins to fall, while the conditions inside the house remain the same. As a result, the heat lost per second increases. What is the temperature at the outside window surface when the heat lost per second doubles?

Answer 1

To conduct twice the heat through the window, the temperature drop across the window glass must double from 20 C to 40 C. That will require that outside temperature becomes -15C

Answer 2

-15C is right but how did you get it?