Three cones of water should fill the glass to the top. The glass is a cylinder with volume base x height. So the volume of a cone = 1/3 base x height.

the bottom edge of the rectangle is 1/2 of the circumference of the base of the cone. The height of the rectangle is the slant length... Image caption, ...you've done it! The sectors rearranged ...

For example, the volume of a pyramid is one-third that of a prism with the same base and height, and the volume of a cone is one-third that of a cylinder with the same base and height. The video ...