Answer :
Using the Newton's law of cooling, the time it takes to cool to 100°F is 116 mins.
Newton's law of cooling
According to the Newton's law of cooling, the when an object is exposed to the surrounding it looses heat rapidly untill it is at a thermal equilibrium with the suurroundings.
Now;
T(t) = T(s) + (To - Ts)e^-kt
T(t) = temperature at time T
To = initial temperature
Ts = temperature of the surrounding
k = colling constant
t = time taken
200 = 68 + (230 - 68)e^-15k
200 = 68 + 162e^-15k
200 - 68/162 = e^-15k
ln(0.815) = -15k
k = ln(0.815)/-15
k = 0.014 min-1
Now;
100 = 68 + (230 - 68)e^-( 0.014t)
100 -68/162 = e^-( 0.014t)
0.198 =e^-( 0.014t)
ln(0.198) = -( 0.014t)
t = ln(0.198)/-0.014
t = 116 mins
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