Since then C measures A according to the units in D, while the unit also measures D according to the units in it,
therefore the unit measures the number D the same number of times as the magnitude C measures A;
therefore, as C is to A, so is the unit to D; [VII. Def. 20]
therefore, inversely, as A is to C, so is D to the unit. [cf. V. 7, Por.]
Again, since C measures B according to the units in E, while the unit also measures E according to the units in it,
therefore the unit measures E the same number of times as C measures B;
therefore, as C is to B, so is the unit to E.
But it was also proved that, as A is to C, so is D to the unit;
therefore, ex aequali, as A is to B, so is the number D to E. [V. 22]