Originally Posted by EvilToast
i don't get this calculation^^
archaic knowledge gives 15% magic armor penetration
void staff gives 40 % magic armor penetration
though u get void staff AFTER the mastery, do you, in total, get 15 % + (15*40%) magic armor penetration? this would equal 21 %. oO
now i am confused xD
Actually, you're thinking about it the wrong way round.
NOTE: I'm probably not the best person to explain this; if you can follow this then you could probably explain it back to me even better.
I think it works in an almost reverse order:
15% penetration = Hitting against 85% of their MR/Armour(note: there are currently no other percentage armour things).
40% penetration = Hitting against 60% of their MR.
85%*60% = 51%
Therefore you are hitting against 51% of their MR/Armour and therefore only penetrating 49%.
I quoted the result from here
and after playing with the numbers that was the only, logical, sense I could make of it.
If somebody else with a better understanding of mathematics could help prove me right/wrong then it would be most appreciated.
EDIT: It may also be worth adding, in order to keep all the info in one thread, that by using items like Abyssal Scepter and Black Cleaver it is possible to reduce your targets MR/Armour to Negative values, however, Magic/Armour Penetration only applies to MR/Armour that is Positive.
In sum: Increasing your MR/Armour reduction will not benefit your Armour/Magic Penetration, in fact quite the opposite, it will weaken it's affect.