the total volume is given by the volume of the hemisphere and the volume of the cylinder. thus, if we let be the total volume

where is the height of the cylinder and is the radius (both the cylinder and hemisphere have the same radius)

now the key is, the total height is 30 feet. note that this height includes the radius of the hemisphere (which is the same as the radius of the cylinder). so that the height of the cylinder is , thus we have

now solve for