Atank initially holds 80 gal of a brine solution containing 1/8 lb of salt per gallon. at t = 0, another brine solution containing 1 lb of salt per gallon is poured into the tank at the rate of 4 gal/min, while the well-stirred mixture leaves the tank at the rate of 8 gal/min. find the amount of salt in the tank when the tank contains exactly 40 gal of solution.