Length × breadth × thickness = volume, or cubic content.
For example, suppose the solid in the diagram is 10 feet in length, 8
feet in breadth, and 5 feet in thickness. It is clear that the solid can
be cut into five slices, each 1 foot thick, by planes parallel to the
bottom. But, the bottom contains 10×8 square feet and above each square
foot there is a cubic foot. Thus, each slice contains 10×8 cubic feet.
Therefore, since there are five slices, the whole solid contains 10×8×5,
or 400 cubic feet.
Since length × breadth × thickness = cubic content, it follows that, if
we know any three of these four quantities, we can find the fourth.