Problem 1 · Differential equations
A casting cools in a 20°C room according to dT/dt = −k(T − 20), with k = 0.05 min⁻¹ and T in °C. Its temperature at t = 0 is 90°C. Enter the temperature at t = 30 min, in °C, to one decimal place.
Fill in the blank: answer in °C.
Show the answer and solution
Answer: 35.6 °C (accepted 35.2 to 36)
Separating variables gives T(t) = 20 + (90 − 20)e^(−kt). At t = 30 min: T = 20 + 70e^(−1.5) = 20 + 70(0.2231) = 35.6°C. Applying the decay to 90°C instead of to the 70°C excess over ambient gives 20.1°C.
Differential equations · Reference Handbook: Mathematics
