1. The problem statement, all variables and given/known data A tank contains 80 gallons of pure water. A brine solution with 2 lb/gal of salt enters at 2 gal/min, and the well-stirred mixture leaves at the same rate. Find (a) the amount of salt in the tank at any time and (b) the time at which the brine leaving will contain 1 lb/gal of salt. 2. Relevant equations I'm just wondering about (b) really. I know we set S=80 below to solve it, but why? 3. The attempt at a solution The differential equation that gives (a) is S=160 - 160*e^(-t/40) where S is the amount salt in the tank at any time t.