Check the picture below
so.. if you notice, the diameter is 20, the radius is half that, the radius and height are at a ratio of 5:6, if the larger cone is filled up to 9ft, that means, the conical gap above, is 3ft in height
so, simply getting the volume of the larger cone filled up, with a height of 12, and then subtracting that conical gap, you'd end up with what's leftover, whatever the volume is at 9ft
so.. if you notice, the diameter is 20, the radius is half that, the radius and height are at a ratio of 5:6, if the larger cone is filled up to 9ft, that means, the conical gap above, is 3ft in height
so, simply getting the volume of the larger cone filled up, with a height of 12, and then subtracting that conical gap, you'd end up with what's leftover, whatever the volume is at 9ft