The mathematical model of the demand for a certain car is given by x,y)=36000 x+ l 0.9yl2 & 4 25000 SB where I represents the price per car in RM, y represents the price of petrol per liter, and D is the demand for cars. The price of the car and price of the petrol after 1' years from now is given by the following equations x = 72000+400t, y 2 480+ 20; Find the rate of change of the demand in 10 years. [8 Marks]