Two metal boxes $A$ and $B$ with heat capacity $C$ and $2 C$ respectively are interconnected by a rod of length $l$, cross-sectional area $A$ and Thermal conductivity $k$. At $t=0$, temperature of $A$ and $B$ is $40°\mathrm{C}$ and $70°\mathrm{C}$ respectively. Find time $t$ when temperature of $A$ is $50°\mathrm{C}$.

A brass rod and an aluminium rod each of length $1 \mathrm{m}$ and same cross section area $1{ \mathrm{cm}}^2$ are placed in contact with each other as shown. Temperature of both the rods is raised by $100 K$. The coeffcients of linear expansion of brass and aluminium are $2.0\times {10}^{-5}{ \mathrm{K}}^{-1}$ and $2.4\times {10}^{-5}{ \mathrm{K}}^{-1}$ respectively. If the Young's moduli of brass and aluminium are ${10}^{11} \mathrm{N} {\mathrm{m}}^2$ and $7\times {10}^{10} \mathrm{N} {\mathrm{m}}^2$ respectively then stress in the rods is

The figure shows the face and interface temperature of a composite slab consisting of four materials, of identical thickness, through which the heat transfer is steady. Which material has maximum thermal conductivity?