I just realized that I have never used something as simple as proportions in spoken English so far, but today I have to. So simple question for native speakers: How do you pronounce for example $$ a:b=c:d$$ where the colon denotes a proportion rather than a division? (For example, in German we'd say "$a$ zu $b$ wie $c$ zu $d$" for proportions as opposed to "$a$ durch $b$ gleich $c$ durch $d$" for divisions).
"$a$ is to $b$ as $c$ is to $d$."
"The ratio of $a$ to $b$ is equal to the ratio of $c$ to $d$."
On a rather pedantic note, this isn't an issue of "pronunciation" per se, but one of "verbalization." How do we speak or verbalize a mathematical construct (in this case, an equation between proportions).
In English, we pronounce $a:b :: c:d$ as "a is to b as b is to c", where the colon in particular is pronounced "is to".
The $::$ sign that I have used in place of the equality sign is the proportionality sign.
So if you wish to say $a:b = c:d$, then you will say "a is to b is equal to b is to c".