I've run into a problem with armor penetration mechanics.

Apparently, armor reduction runes + mastery cant reduce armor below 0.
On the contrary, armor penetration from items can.
So my question is which will be taken into consideration first?

Assuming the target has 30 armor.
From runes + mastery, I have 25 armor reduction.
I also have a brutalizer (15 armor penetration) and a last whisper (40% armor penetration).

What will be the final armor value of my target in this case?
I would like to know the order of which those effects take place.

Ty

I believe I read in another thread that any armor pen that is a flat # (Spirit Fire, Brutilizer, Runes, Mastery) are applied and then anything that is a % is applied (Last Whisper.....)

Don't forget it works on towers.

only flat #'s effect towers i think one sec finding thread.

bah can't find it, may have been in guides and strats but it had lots of good info in it.

Pretty sure the deal was all Armor Penetration can't go below 0, but armor debuffs put on the enemy can bring them below zero. So in the example:

30-12-15-25 = 0

If you go for a black cleaver instead of the Brutalizer/Last Whisper with your armor pen runes, you'd end up with:
30-25 = 5
5-60 = -55

Okay, here is the official way that various penetration effects work:

First, apply any Reduction effects, like Taric's Shatter Active, Soraka's Starcall, Black Cleaver's Stack, Fiddlesticks' Aura, etc. This CAN go negative.
Second, apply any percentage-based Penetration effects (Last Whisper, Archaic Knowledge, Void Staff). This clearly cannot go negative, because the highest you can get is 55%.
Lastly, apply any flat Penetration Effects (Sunder Mastery, Runes, Brutalizer, Sorc Boots, Haunting Visage, etc). This cannot go below 0.

Ah my bad, so the final value would be -30 with the cleaver and -25 in armor pen runes, with the runes effectively giving no benefit vs such a low armor target.

Well the runes early game would neglect the starting armor, and into the game the runes + Cleaver would bring most non tanks back to zero, i defiantly think armor, and magic penatration is worth it. i think people get bummed out that it cant bring it into the negative, but it its still evry much worth it.