A hot body cools from 90 degree celcius to 80 degree celcius in 5 minutes when placed in a room of temperature 30 deg C. How long will it take to cool from 70 deg C to 60 deg C in the same room ?

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According to Newton's law of cooling, heat lost per second by the body is proportional to mean excess of temperature of the body above the surroundings.

If m is the mass ...

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(Please see the attachment for detailed problem description)
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