Question

Hot water cools from 60°C to 50°C in the first 10 minutes and to 42°C in the next 10 minutes. The temperature of the surroundings is: 

(a) 25°C 

(b) 10°C 

(c) 15°C 

(d) 20°C

Answer 1

Correct option: (b) 10°C

Explanation:

By Newton’s law of cooling

where θ₀ is the temperature of surrounding. Now, hot water cools from 60°C to 50°C in 10 minutes,

Again, it cools from 50°C to 42°C in next 10 minutes.

Dividing equations (i) by (ii) we get