Two tanks are interconnected. Both tanks initially contain 90 L of salt solution. The first tank initially contains 3 kg of dissolved salt and the second tank initially contains 2 kg of dissolved salt. Pure water enters the first tank at a rate of 4.5 Liters per minute. Salt solution from the first tank flows into the second tank at a rate of 6 Liters per minute and salt solution from the second tank is pumped back into the first tank at a rate of 1.5 Liters per minute. Salt solution leaves the system through the second tank at a rate of 4.5 Liters per minute.

Represent the situation described above as a linear system of differential equations with initial conditions. Use Q1​ for the mass of salt in the first tank after t minutes and Q2 for the mass of salt in the second tank after t minutes.