The wall of a refrigerator consists of four layers. The first and third have a thickness of 5 cm and a thermal conductivity of 
, while the second and fourth have a thickness of 2 cm and a thermal conductivity of 
. The first layer encloses the interior of the refrigerator with a temperature of 
, and the fourth layer is outside, where the temperature is 
. What is the temperature 
in the middle of the third layer?
