For the "closed" system of three brine tanks with volumes V1 = 50 gal, V2 = 25 gal, V3 = 50 gal, and constant flow rate r = 10 gal/min, find the amounts of salt x1(t), x2(t), and x3(t) at time t in the three brine tanks, if the initial amount in each of the tanks is: x1(0) = 50, x2(0) = x3(0) = 0. (The difference between this system and "open" system is that now the inflow to tank 1 is the outflow from tank 3.)