nevermind...

producing the same Z amount WITHOUT speed modules and beacons, but with a larger amount of assembly machines?
This is strictly better, because you have a higher productivity ratio in every assembling machine. It just costs twice as much to set up.

Once you have your giant, slow, very productive ...

Sorry, I actually didn't see your first comment in your post (before the quote) saying that the first formula is correct. Thank you.

about my second question:
What I mean with "the math behind it" is: Why does this formula imply that it is better/more cost effective/better return on investment, to ...

Yes the first formula is correct and it's additive.

In any case, I have read that "speed modules and productivity modules complement eachother". Can someone show the math behind this?
Why is it better to use 1 speed module and 1 productivity module, rather than 2 of either? (i.e. what is the math ...

This is such a simple question that it may be obvious to everyone, but to me it isn't.

the productivity module 1 gives the following:
speed -15%
productivity bonus 4%

What EXACTLY is the formula that determines production here?

Assume we're making copper cable in an assembly 2 machine:
Assembly ...