A cup of coffee cools from 90°C to 80°C in t minutes, when the room temperature is 20°C. The time taken by a similar cup of coffee to cool from 80°C to 60°C at a room temperature same at 20°C is :

CONCEPT:

 According to Newton's Law of Cooling, we can find out the time taken by the cup, we have;

Where T1 and T2 are the initial and final temperature and Ts is the surrounding temperature, and Δt is the time taken and k is a positive constant depending upon the area and nature of the surface of the body.

CALCULATION:

By using Newton's law of cooling, 

For the first case we have;

Given: Initial Temperature, 

Final temperature, 

and surrounding temperature, 

Now, on putting the above values in equation (1) we get;

For the second case we have;

Similarly, 

Given: Initial Temperature, 

Final temperature, 

and surrounding temperature, 

where t' is the time taken in the second case

substituting the equation (2) in (3) we get;

Hence, option 3) is the correct answer.