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An object kept in a large room having air temperature of 25°C takes 12 minutes to cool from 80°C to 70°C. The time taken to cool for the same object from 70°C to 60°C would be nearly.

1. 15 min
2. 10 min
3. 12 min
4. 20 min

Correct answer: 15 min

Solution

According to Newton's law of cooling, the rate of cooling is proportional to the temperature difference between the object and its surroundings. As the average temperature difference in the second interval (70°C to 60°C) is smaller than in the first interval (80°C to 70°C), the cooling will take slightly longer. Hence, the time is approximately 15 minutes.

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