A question about armor shred/ignore and relevant percents

So is it multiplicative or additive or how does it work?

For example: Darius with maxed E (25% armor ignored), Black Cleaver (30% shred), and just for the hell of it, lord dominik/mortal reminder (45% ignore). How would all those %'s stack? Would E and dominik/mortal stack for a straight 70%, or do some weird math for a different number? And how would Black Cleaver affect that number?

Just for some arbitrary number to work with, let's say the target has 200 armor, and we have 0 lethality, so we're working purely with these %'s.