A composite slab consists of two slabs $\mathrm{A}$ and $\mathrm{B}$ of different materials but of the same thickness placed one on top of the other. The thermal conductivities of $\mathrm{A}$ and $\mathrm{B}$ are $k_1$ and $k_2$ respectively. A steady temperature difference of $12 °\mathrm{C}$ is maintained across the composite slab. If $k_1=\frac{k_2}{2},$ the temperature difference across the slab $\mathrm{A}$ will be