A tank is filled with $V=200L$ of a brine containing $=\mathrm{.}4kg$ of salt A per litre. At moment $0,$ input and output valves are opened, and a brine containing another salt $B,$ with concentration $=\mathrm{.}2kg$ per litre runs into the tank at a rate $r_i=5\frac{L}{sec}.$ The mix runs out of the tank with rate $r_0=4\frac{L}{sec}.$ The salts do not interact with each other. Determine the ratio $k$ of quantity of salt $B$ to the quantity of salt A when the tank contains $V_1=400L_1$ of the mixture