Salt water pours into a full 20 liter tank at a rate of 21L/min. the inflow initial Concentration is 2kg/L but after 30 minutes the inflow Concentration Switches to 3 kg/s; the braine in the tank is well mined and it drains out at a cave of 2 //min. Call y the kilograms of Salt in the tank at a time t. The tank is initially full of fresh water. Solve the differential equation that models the salt in the tank?