A100 gallon tank is initially half full of pure water. a spigot is opened above the tank and a salt solution containing 0.1 lb of salt per gallon of solution begins flowing into the tank at a rate of 4 gal/min. simultaneously, a drain is opened at the bottom of the tank allowing the well stirred contents of the tank to leave the tank at a rate of 2 gal/min. express the amount of salt in the tank as a function of time t.