If two numbers multiplying some number make some other numbers then the numbers generated from them will have the same ratio as the multiplying numbers.
For let the two numbers A and B make the numbers
D and E respectively, by multiplying some number C.
I say that as A is to B, so D is to E.
For since A has made D by multiplying C, C has
thus also made D by multiplying A [Prop. 7.16]. So, for
the same reasons, C has also made E by multiplying
B. So the number C has made D and E (by) multiplying
the two numbers A and B (respectively). Thus, as A is to
B, so D is to E [Prop. 7.17]. Which is the very thing
it was required to show