A body takes 4min to cool from 61 ° C to 59 ° C . If the...

\dfrac{\DeltaT}{\Deltat} = K\left({T}_{t} - {T}_{S}\right){T}_{t} =$average temperature</p><p>${T}_{S} =$surrounding temperature</p><p>$\dfrac{61 - 59}{4} = K\left(\dfrac{61 + 59}{2} - 30\right)$. . . ( i )$

$\dfrac{51 - 49}{t} = K\left(\dfrac{51 + 49}{2} - 30\right) . . . \left(ii\right)$

Divide $( 1 )$ and $( 2 )$

$\dfrac{t}{4} = \dfrac{60 - 30}{50 - 30} = \dfrac{30}{20}$

So, $t = 6$ $min$