1. The problem statement, all variables and given/known data The average rate at which energy is conducted outward through the ground surface in North America is 54 mW/m^2, and the average thermal conductivity of the near-surface rocks is 2.5 W/mK. Assuming a surface temperature of 10 C, find the temperature at a depth of 35 km. Ignore the heat generated by the presence of radioactive elements. 2. Relevant equations P = kA(Th-Tc)/L where P is the conduction rate k = thermal conductivity A = face area L = thickness Th = temperature of hot reservoir Tc = temperature of cold reservoir 3. The attempt at a solution I can plug all values into the equation for conduction rate except for A. Is there something I'm missing in terms of the surface area for this problem? In the answers I've seen online, people seem to assume A is 1m^2. However, I can't figure out why that assumption is made. (The answer seems to be 766 C, which is found using A=1m^2). Thanks for your help!