Solution:
Solution:
From Newton’s law of cooling,
$$\frac{d\theta}{dt} \propto \left(\theta - \theta_{0}\right)$$
in case (i) $$ \frac{94 - 86}{2} \propto \left(\frac{94 + 86}{2} - 20\right)$$
$$ \Rightarrow \frac{8}{2} \propto \left(90 -20\right) or 4 \propto 70$$
in case (ii) $$ \frac{74 - 66}{t} \propto \left(\frac{74 + 66}{2} -20 \right)$$
$$ \frac{8}{t} \propto \left(70 - 20 \right)$$
$$ \frac{8}{t}\propto 50 $$
$$Dividing \left(1\right) and \left(2\right)$$
$$ \frac{t}{2} = \frac{70}{50} $$
$$ t \frac{14}{5} $$
t = 2.8 minutes
