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Frustum-Shaped Mound

There is a mound of earth in the shape of a frustum of a cone. The circumferences of the bases are 40 measures and 120 measures and the mound is 6 measures high. If 1200 cubic measures of earth are removed evenly from the top, what will be the remaining height?

Jinkō-ki (Treatise on Large and Small Numbers), Yoshida Kōyū, 1627

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"Frustum-Shaped Mound," Convergence (June 2007)