Coffee is poured at a uniform rate of 20 cm^3/s into a cup whose inside is shaped like a truncated cone. If the upper and lower radii of the cup are 4 cm and 2 cm and the height of the cup is 4 cm, how fast will the coffee level be rising when the coffee is halfway up? [ hint: Extend the cup downward to form a cone.]

i know that v = (1/3)(r^2)(h)

i dont know how to set r as a function of h or vice versa.