For, since A is composite, it will be measured by some number. [VII. Def. 13]
Let it be measured by D;
and, as many times as D measures A, so many units let there be in E.
Since then D measures A according to the units in E,
therefore E by multiplying D has made A. [VII. Def. 15]
And, since A by multiplying B has made C, and A is the product of D, E,
therefore the product of D, E by multiplying B has made C.
Therefore C is solid, and D, E, B are its sides.
Q. E. D.