Alarge tank contains 200 liters of a salt solution which has a concentration of 0.35 kilograms per liter. pure water is added to the solution at a rate of 40 liters per minute. at the same time, the solution drains from the tank at 40 liters per minute

The question is incomplete, but from the given information it's clear that this is a standard mixing problem. Let be the amount of salt (in kilograms) in the tank at time .

Then the rate of change of the amount of salt in the tank is described by the differential equation




Separating the variables, we get



and integrating both sides yields




Initially, the tank contains of salt, i.e. . This means you have



and so the amount of salt in the tank as a function of time is