$\dfrac{dT}{dt} = -k(T - T_s)$
$\dfrac{dT}{dt} = -k(T - 20)$
$\dfrac{dT}{T - 20} = -k\,dt$
$\ln (T - 20) = -kt + \ln C$
$\ln (T - 20) = \ln e^{-kt} + \ln C$
$\ln (T - 20) = \ln Ce^{-kt}$
$T - 20 = Ce^{-kt}$
$T = 20 + Ce^{-kt}$
When t = 0, T = 75°
$75 = 20 + Ce^{0}$
$C = 55$
Hence,
$T = 20 + 55e^{-kt}$
When t = 4, T = 30°
$30 = 20 + 55e^{-4k}$
$\frac{10}{55} = e^{-4k}$
$e^{-k} = \left( \frac{2}{11} \right)^{1/4}$
Thus,
$T = 20 + 55\left( \frac{2}{11} \right)^{t/4}$
When T = 20.5°F
$20.5 = 20 + 55\left( \frac{2}{11} \right)^{t/4}$
$\dfrac{0.5}{55} = \left( \frac{2}{11} \right)^{t/4}$
$\ln \dfrac{0.5}{55} = \ln \left( \frac{2}{11} \right)^{t/4}$
$\ln (\frac{1}{110}) = \frac{1}{4}t \, \ln (\frac{2}{11})$
$t = \dfrac{4\ln (\frac{1}{110})}{\ln (\frac{2}{11})}$
$t = 11.029 ~ \text{minutes}$ answer
Comments
Paano po nakuha yung 20.5 na
Paano po nakuha yung 20.5 na value ng T?
... the temperature is 20
... the temperature is 20 degree... within half degree of the air temperature