3. A small metal bar, whose initial temperature was 20° C, is dropped into a large container of bowling water. How long will it take the bar to reach 90°C if it is known that its temperature increase to 22°C in 1 second? 4. A tank 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
