question_answer

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:                                                                                                         [JEE ONLINE 12-04-2014]

A)  25°C   

B)       10°C

C)  15°C   

D)       20°C

Correct Answer: B

Solution :

[b] By Newton’s law of cooling

where  is the temperature of surrounding.

Now, hot water cools from 60°C to 50°C in 10 minutes, .(i)

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

Dividing equations (i) by (ii) we get