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since volume scales as the cube of the linear scale,
the new cone is scaled in every dimension to 3/4 the original cone. So, its volume is (3/4)^3 * 1920 = 810
Note that you don't need the height, just the relative scale factor.
1/3 pi r^2 h = 1920
1/3 pi (3/4 r)^2 (3/4 h) = 1920 * (3/4)^3