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Question

A body takes 4 minutes to cool from 100oC to 70oC. The time taken by it to cool from 70oC to 40oC in a room with temperature 15oC is nearly_____

A
4 minutes
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B
5 minutes
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C
6 minutes
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D
7 minutes
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Solution

The correct option is D 7 minutes
The Newtons law of cooling formula is given by:
T(t)=Ts+(ToTs)ekt
T(t)TsToTs=ekt
kt=lnT(t)TsToTs
kt=ln701510015
Thus we get k=0.10883
Now for the second interval we get:
kt=ln40157015
Thus, t=7 mins

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