For, since A is composite, it will be measured by some number. [[VII. Def. 13](elem.7.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](elem.7.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.