Answer is supposed to be -0.036 kg / m³ * sec. I need steps please!

Air enters the lungs through a circular channel with a diameter of 3 cm and a velocity of 150 cm/s and a density of 1.25 kg/m. Air leaves the lungs through the same opening at a velocity of 120 cm/s and a density equal to that of the air within the lungs, which has decreased its net density, due to the lower density air entering the lungs. At the initial conditions the air within the lungs has a density of 1.4 kg/m, with a total volume of 6 L. Find the aggregate rate of change of the density of air in the lung assuming that your time step includes one inhale and one exhale (takes 15 s) and that there is no change in the lung volume over this time