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Volume of the stone?

A normal-level "Volume" problem.However complicated the shape, sinking it in water measures its volume — the very thing Archimedes noticed in his bath.

GivenA tank with a 20 cm by 15 cm base. Dropping the stone in raised the water by 3 cm

Volume ★★★☆☆☆ Volume of the stone? 20cm 15cm 3cm

Hints

  1. Not a single drop of water was added when the stone went in — the amount of water has not changed at all.
  2. And yet the surface rose by 3 cm. The water in that red slab is water pushed out of the way by the stone, with nowhere to go but up.
  3. In other words the stone's volume equals the volume of the risen water. The stone's lumpy shape is completely irrelevant.
  4. Clear everything else and look at the risen slab alone: a cuboid with the tank's own 20 cm × 15 cm base and a height of 3 cm.
  5. Now it is just a cuboid: 20 × 15 × 3

Answer 900cm³

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