Answer:Omg JUST use slope intercept form like I said: Pick a point in this case I have picked, but you can any: (15, 560) (20, 480). Remember
Y2 - Y1 /X2-X1 = M (Your slope). In this case 480-560/20-15 = -80/5 = -16
Now you have your slope. Y - Y2=M(x - x1). In this case Y - 560 = -16(x-15)
Distrubute: Y - 560 = -16x +240. Add 560 on both side. Y= -16x +800
And yes it water decreases as time increases.
Explanation: