Models & Qualitative Behavior · Superchallenge problem

A tank holds $100$ L of pure water. Brine with $2$ g/L of salt flows in at $5$ L/min, and the well-mixed solution drains at $5$ L/min. The salt amount satisfies $\dfrac{dS}{dt} = 10 - \dfrac{S}{20}$. How many grams of salt does the tank approach as $t \to \infty$?