Question

A liquid takes $5$ min. to cool from ${80}^{\circ }C$ to ${50}^{\circ }C$. How much time does it take to cool from ${60}^{\circ }C$ to ${30}^{\circ }C$?Temperature of surroundings is ${20}^{\circ }C$.

• A. 2 mins
• B. 3 mins
• C. 5.5 mins
• D. None of these

Answer 1

The correct option is D None of these

Let the required time be $x$ min.

$\frac{\frac{(80-50)}{5}}{\frac{30}{x}}=\frac{\{\frac{80+50}{2}-20\}\times k}{\{\frac{60+30}{2}-20\}\times k}\Rightarrow \frac{6}{\frac{30}{x}}=\frac{\{65-20\}k}{\{45-20\}k}\Rightarrow 6\times \frac{x}{30}=\frac{45}{25}\Rightarrow x=9min$