A body cools from $80^\circ\text{C}$ to $60^\circ\text{C}$ in $5\text{ minutes}$ in surroundings at $20^\circ\text{C}$. Find time taken to cool from $60^\circ\text{C}$ to $40^\circ\text{C}$.
2.
Two rods of equal length $L$ and area $A$ with thermal conductivities $K_1$ and $K_2$ are connected in series. Derive the effective thermal conductivity $K_{eq}$.
