First-Order Equations · Medium problem

A cup of coffee cools according to Newton's law $\dfrac{dT}{dt}=-k(T-20)$ in a $20^\circ\mathrm{C}$ room. It is $100^\circ\mathrm{C}$ at $t=0$ and $60^\circ\mathrm{C}$ at $t=10$ minutes. What is its temperature, in $^\circ\mathrm{C}$, at $t=20$ minutes?