Originally Posted by Vuther
The order of the calculation is flat armour reduction, then % armour reduction, then % armour penetration, and flat armour penetration last.
100 X (0.65 [Last Whisper]) X (0.75 [Black Cleaver]) - 20 (Youmuu's Ghostblade) = 28.75 actual armour against you.
You made one mistake:
100 x .75 = 75 for BC reduction
75 x .65 = 48.75 for LW penetration
48.75 - 10 - 20 = 18.75 for the BC and YG penetration. You had forgotten the BC has 10 flat.
You could have another -8% and flat -24 pen from runes and masteries.
100 armor = 0
actually, 126 armor = 0!
but who is building armor nowadays? League of Warmogs