A closed cubical box is made of perfectly insulating material and the only way for heat to enter or leave the box is through two solid cylindrical metal plugs, each of cross sectional area 12cm^2 and length 8cm fixed in the opposite walls of the box. The outer surface of one plug is kept at a temperature of 100C . while the outer surface of the plug is maintained at a temperature of 4C . The thermal conductivity of the material of the plug