Models & Qualitative Behavior · Easy problem

A metal rod cools in a $20^\circ$C room according to Newton's law of cooling $\dfrac{dT}{dt} = -\dfrac{1}{10}(T - 20)$, with $t$ in minutes. At what rate (in degrees per minute) is its temperature changing at the moment the rod is at $50^\circ$C? Give a signed answer.