Two rectangular blocks, having indentical dimensions, can be arranged either in configuration $I$ or in configuration $II$ as shown in the figure, On of the blocks has thermal conductivity $k$ and the other $2 \ k$. The temperature difference between the ends along the $x$-axis is the same in both the configurations. It takes $9\ s$ to transport a certain amount of heat from the hot end to the cold end in the configuration $I$. The time to transport the same amount of heat in the configuration $II$ is :
IIT 2013, Diffcult
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In configuration $1$ equivalent thermal resistance is $\frac{3 R}{2}$
In configuration $2$ equivalent thermal resistance is $\frac{R}{3}$ Thermal Resistance $\propto$ time taken by heat flow from high temperature to low temperature
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