A body cools from 500C to 49oC in 5 sec. How long will it take to cool 400C to 39.00C? Assume the temperature of surroundings to be 300C  and Newton's law of cooling to be valid:

According to Newton's law of cooling, the ratio of cooling directly proportional to the temperature difference. When the temperature difference is halved, the rate of cooling is also halved. So time taken is 10 sec.