Question

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 (room temperature is $15^{o}C$)

• A. 7 minutes
• B. 6 minutes
• C. 5 minutes
• D. 4 minutes

Answer 1

Answer: (B)

6 minutes

Answer 2

$\frac{\theta_{1} - \theta_{2}}{t} = K\left( \frac{\theta_{1} + \theta_{2}}{2} - \theta_{0} \right)$

$\therefore$ $\frac{100 - 70}{4} = K\left( \frac{100 + 70}{2} - 15 \right)$ = 60K ⇒ K = $\frac{1}{8}$

Again $\frac{70 - 40}{t} = \frac{1}{8}\left( \frac{70 + 40}{2} - 15 \right)$= 5 ⇒ t = 6 min.