Armor Penetration Discussion

I know this may be a dated question and be answered in the archived forum, but I had trouble finding it. I'm curious as to what happens when a champion has Percent armor reduction and uses a black cleaver? Say a champion like Vi has a 20% armor reduction on her W and Black Cleaver has 5% reduction per hit up to 25%, does this make the armor reduction a)45% or b)20% then 25%(or vice versa multiplication is communicable). It seems odd, because 45% armor shred just seems ridiculous and immensely helpful to the team's ADC / AD Assassin

1)Just what I was thinking while I faced a 305 armored Malphite was that with 25% reduce,20% reduce, 6%pen, and 30 flat pen: a1)305 * 0.55 = 167.75 * 0.94 = 157.685 - 30 = 127.685 armor/56.08% reduced damage b1)305 * 0.80= 244 * 0.75 = 183 * 0.94 = 172.02 - 30 = 142.02 armor/58.68% reduced damage

2)Then factor in his level 5 Brutal Strike 40% increase: a2)427 * 0.55 = 234.85 * 0.94 = 220.759 - 30 = 190.759 armor/65.61% reduced damage b2)427 * 0.80 = 341.6 * 0.75 = 256.2 * 0.94 = 240.828 - 30 = 210.828 armor/67.83% reduced damage

I don't know why I'm still writing this or thinking about posting this, because I'm basically asking the difference between 1) 2.6% damage reduction and 2) 2.22%. Matter of fact when it comes down to those small numbers if I had the ~285 Attack damage I had the difference is legit 1) 7.41 per basic attack and 2) 6.327. I can't really even see this becoming a huge factor for ADC's who crit. all the time even. Say 340 atck. dmg. on an ADC 2.5 crit. damage is 875 crit. 1) 22.75 loss per crit. and 2) 19.425 loss per crit.

Though 2 attack per second, 50% crit chance, difference of 1) 31.59 damage per second and 2) 26.905 per second... against a tank which you are not hitting.

TL;DR Does % Armor Reduction stack or is it multiplied consecutively? ( Useless ranting of about numbers that are more than likely going to change come Season 5 and if not don't make a huge difference any way)