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Question

A pan filled with hot food cools from 94C to 86C in 2 minutes. When the room temperature is 20C. How long will it cool from 74C to 66C?

A
2 minutes
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B
2.8 minutes
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C
2.5 minutes
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D
1.8 minutes
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Solution

The correct option is C 2.8 minutes
Using Newton's law of cooling with approximation, dTdt=kms(TavgTs)
Case 1 : dt=2 minutes; Ts=20
Tavg=94+862=90 and dT=9486=8
kms=dTdt.1TavgTs=82×70=0.05714 .......(a)
Case 2 : Ts=20
Tavg=74+662=70 and dT=7466=8
Time in which it cools down 8dt=0.05714×(7020)
dt=2.8 minutes

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