suppose a tank contains 10l of brine solution (salt dissolved in water). assume the initial concentration of salt is 100g/l. another brine solution flows into the tank at a rate of 3l/min with a concentration of 400g/l. suppose the mixture is well stirred and flows out of the tank at a rate of 3l/min. let y(t) denote the amount of salt in the tank at time t. find y(t).