Application of Differential Equation: Newton's Law of Cooling

$T = T_s + (T_o - T_s)e^{-kt}$
 

At 2:00PM
$T = 20 + (80 - 20)e^{-kt}$

$T = 20 + 60e^{-kt}$
 

At 2:03PM
$42 = 20 + 60e^{-k(3)}$

$e^{-3k} = \frac{11}{30}$

$e^{-k} = \left( \frac{11}{30} \right)^{1/3}$
 

Hence,
$T = 20 + 60\left( \frac{11}{30} \right)^{t/3}$
 

t₁ minutes after 2:03, the thermometer reads
$T = 20 + 60\left( \frac{11}{30} \right)^{{t_1}/3}$
 

When the thermometer is brought back to the room
$T = 80 + (T_o - 80)\left( \frac{11}{30} \right)^{t/3}$

$T = 80 + \left[ 20 + 60\left( \frac{11}{30} \right)^{{t_1}/3} - 80 \right] \left( \frac{11}{30} \right)^{t/3}$

$T = 80 + \left[ 60\left( \frac{11}{30} \right)^{{t_1}/3} - 60 \right] \left( \frac{11}{30} \right)^{t/3}$

$T = 80 + 60 \left[ \left( \frac{11}{30} \right)^{{t_1}/3} - 1 \right] \left( \frac{11}{30} \right)^{t/3}$
 

At 2:10PM, t = 7 - t₁
$71 = 80 + 60 \left[ \left( \frac{11}{30} \right)^{{t_1}/3} - 1 \right] \left( \frac{11}{30} \right)^{(7 - t_1)/3}$

$-9 = 60 \left[ \left( \frac{11}{30} \right)^{t_1/3} - 1 \right] \dfrac{\left( \frac{11}{30} \right)^{7/3}}{\left( \frac{11}{30} \right)^{t_1/3}}$

$-9 = 60 \left[ \left( \frac{11}{30} \right)^{7/3} - \dfrac{\left( \frac{11}{30} \right)^{7/3}}{\left( \frac{11}{30} \right)^{t_1/3}} \right]$

$\dfrac{\left( \frac{11}{30} \right)^{7/3}}{\left( \frac{11}{30} \right)^{t_1/3}} = \left( \frac{11}{30} \right)^{7/3} + \dfrac{9}{60}$

$\dfrac{\left( \frac{11}{30} \right)^{7/3}}{\left( \frac{11}{30} \right)^{7/3} + \dfrac{9}{60}} = \left( \frac{11}{30} \right)^{t_1/3}$

$\left[ \dfrac{\left( \frac{11}{30} \right)^{7/3}}{\left( \frac{11}{30} \right)^{7/3} + \dfrac{9}{60}} \right]^3 = \left( \frac{11}{30} \right)^{t_1}$

$\ln \left[ \dfrac{\left( \frac{11}{30} \right)^{7/3}}{\left( \frac{11}{30} \right)^{7/3} + \dfrac{9}{60}} \right]^3 = \ln \left( \frac{11}{30} \right)^{t_1}$

$\ln \left[ \dfrac{\left( \frac{11}{30} \right)^{7/3}}{\left( \frac{11}{30} \right)^{7/3} + \dfrac{9}{60}} \right]^3 = t_1 \ln \left( \frac{11}{30} \right)$

$t_1 = 2.8 ~ \text{min}$
 

time = 2:05:48.6PM