For a torus (like a bicycle inner tube), Pappus's Centroid Theorem gives the volume as the product of the cross-sectional area and the distance traveled by the centroid of that cross-section as it sweeps around the central axis — equivalently, (cross-sectional area) × (mean circumference of the torus).
The tube's circular cross-section has diameter 6 cm, so radius r = 3 cm. Cross-sectional area = π r² = π × 3² = 9π...
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