Consider a tank with a capacity of 1000 ` and initially contains 400 ` of solution with 200 g of dissolved salt. Starting at time t = 0, brine containing 3 g of dissolved
salt per liter flows into the tank at the rate of 6 `/min. The mixture is kept uniform
by stirring and the well-stirred mixture flows out at a slower rate 2 `/min.
(a) Find a formula gives the amount of the salt in the tank at any time t.
(b) Find the concentration (in grams per liter) of salt in the tank when it is on the
point of overflowing