For this problem A is the amount of salt in the tank. If a tank contains 400 liters of liquid with 17 grams of salt. A mixture containing 10 grams per liter is pumped into the tank at a rate of 2 liters/minute. The mixture is well-mixed, and pumped out at a rate of 5 liters/minute. The amount of salt in the tank satisfies the differential equation = 0. Rewriting this as a linear differential equation we get = 20. The integrating factor is and the solution is If you are having trouble getting the system to accept the final solution, try rounding your C value to exactly six decimal places (starting with the first nonzero decimal place). --KT