Ice starts forming in lake with water at ${0^o}C$ and when the atmospheric temperature is $ - {10^o}C$. If the time taken for $1 \;cm$ of ice be $7$ hours, then the time taken for the thickness of ice to change from $1\; cm$ to $2\; cm$ is
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(d) $t = \frac{{\rho L}}{{2K\theta }}(x_1^2 - x_2^2)$
==>$t \propto (x_2^2 - x_1^2)$
==> $\frac{t}{{t'}} = \frac{{(x_2^2 - x_1^2)}}{{(x{'_2}^2 - x{'_1}^2)}}$
==> $\frac{7}{{t'}} = \frac{{({1^2} - {0^2})}}{{({2^2} - {1^2})}}$
==> $t' = 21\,hours$
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