# Two slabs A and B of equal surface area are placed one over the other such that their surfaces are completely in contact. The thickness of slab A is twice that of B. The coefficient of thermal conductiviy of slab A is twice that of B. The first surface of slab A is maintained at $100^{\circ}C$, while the second surface of slab B is maintained at $25^{\circ}$. The temperature at the contact of their surfaces is

$\begin {array} {1 1} (1)\;15^{\circ}C & \quad (2)\;45^{\circ}C \\ (3)\;55^{\circ}C & \quad (4)\;85^{\circ} \end {array}$