You're asking for some math as well, so here goes:

First, % penetration works first. So if I have a Void Staff/Last Whisper, I pretend your MR/Armor is only 65% of what it really is. Then I factor in things like Brutalizer/Sorc Shoes/Haunting Guise. Those things work additively. So for example, if you had 200 MR and I had Void/Sorc/Haunting, Void reduces 200 to 130, then Sorc/Haunting reduce that 130 to 100.

So instead of dealing 1/3 damage (fighting 200 MR) you deal 1/2 damage (fighting 100 MR). Thus, the passives result in you dealing 50% more damage than before.

Note that penetration can only go so far. What I mean is that, if you have exactly 100 flat magic pen, it will only give you at most 100% bonus damage (your opponent has 100 MR). If your opponents have any more or less than 100 magic resist, you will get slightly lower returns.

Example: You have 100 MR, so I deal half damage. With 100 magic pen, I deal full damage, aka double what I would have before. If you have 200 MR, I deal 1/3 damage. I reduce that with my 100 magic pen to 100 MR, and I now deal 1/2 damage, so 50% more than before. If you have 50 MR, I deal 2/3 damage. With 100 magic pen, I deal full damage, or 50% more than before.

I hope those all made sense.

Quote:

Originally Posted by

**Tesla23**
they work like this:

tank spends +10k on tanky stats.

squishy gets a single 3k item.

Tank now has no stats

Mathematically this isn't true; they roughly equal out. If you had 80 base armor and bought Thornmail, and I bought Last Whisper, your armor went from 80 to 180, then I reduced it to 117. Obviously we each got some extra stats (damage return, 40 AD), but it's pretty easy to see that while % pen is useful, it does not completely devastate tank items.