For, since B is a part of A called by the same name as C, and the unit D is also a part of C called by the same name as it,
therefore, whatever part the unit D is of the number C, the same part is B of A also;
therefore the unit D measures the number C the same number of times that B measures A.
Therefore, alternately, the unit D measures the number B the same number of times that C measures A. [VII. 15]
Therefore C measures A.
Q. E. D.