A body takes $4$ minutes to cool from ${100^o}C$ to ${70^o}C$. To cool from ${70^o}C$ to ${40^o}C$ it will take ........ $\min.$ (room temperature is ${15^o}C$)
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$\frac{{{\theta _1} - {\theta _2}}}{t} = K\,\left( {\frac{{{\theta _1} + {\theta _2}}}{2} - {\theta _0}} \right)$
$\therefore $$\frac{{100 - 70}}{4} = K\,\left( {\frac{{100 + 70}}{2} - 15} \right) = 60K$
==> $K = \frac{3}{28}$
Again $\frac{{70 - 40}}{t} = \frac{3}{28}\left( {\frac{{70 + 40}}{2} - 15} \right)$ $= 5$
==>$t = 7\; min$
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