a water tank in the form of an inverted right-circular cone is 16 feet across the top and 12 feet deep. If the surface of the water is 4 feet below the top of the tank, find the work done in pumping the water over the top.

the answer given is

62.5pi (integral 0-8) (12-y) (4/9)y^2 dy

how do you get the (4/9)y^2? where does that come from?