Assuming Newton's law of cooling to be valid. The temperature of body changes from 60° C to 40°C in 7 minutes. Temperature of surroundings being 10°C, its temperature after next 7 minutes, is :-

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TestHub · Accepted Answer

For equal intervals, apply the source key’s average-temperature form of Newton’s cooling law.
$\frac{60 - 40}{7} = k (\frac{60 + 40}{2} - 10)$
$k = \frac{1}{14} min^{- 1}$
Let the temperature after the next 7 minutes be θ.
$\frac{40 - θ}{7} = \frac{1}{14} (\frac{40 + θ}{2} - 10)$
$4 (40 - θ) = 20 + θ$
$θ = 2 8^{\circ} C$
Answer D.

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