1. The problem statement, all variables and given/known data A tank initially contains 40 gallons of pure water. A solution with 1 gram of salt per gallon of water is added to the tank at 3 gal/min, and the resulting solution dranes out at the same rate. Find the quantity Q(t) of salt in the tank at time t>0. 2. Relevant equations None. 3. The attempt at a solution I don't know how to set up the differential equation for this problem.