You are pouring a solution of your favorite drink containing a (where a is a constant) lb/gal into a large tank at a rate of r gal/min. The tank initially has b gal of pure water. Assume that the tank has stirrers inside and has a hole on the bottom that drains liquid at the same rate r. Let Q be the concentration inside the tank. Write a differential modeling the situation. (Hint: try drawing this out, how would you define dQ/dt?)