If I buy Blackcleaver and Lastwhisper will I have 60% armor penetration?

no, for both practical and mathematical reasons.

1. Reduction actually reduces resistances. That means BC also lowers Rammus' AD cand Malphite's damage.
2. Penetration always works on the current armor. So if The enemy suddenly got +100 armor, LW would just ignore the buff; it would penetrate 35 of that 100 bonus armor.
3. Thus, BC reduces armor, and then LW penetrates the resulting new current armor.
4. For multiple sources of penetration, they are treated as multiple iterations of current armor. The result is multiplicative scaling..

Weird like tenacity. It stacks multiplicatively not additively. So it will bring you around 50% ish at max stacks of Cleaver.

No, you will have 30% armor reduction (at 6 stack) and 35% armor penetration. It equals to [1 - (1-0.3)*(1-0.35)] *100% = 54.5% Armor Penetration.

To discuss it in tankie meta terms.

I have built both BC and Lastwhisper for 3000 + 2300 = 5300 gold

My tankie dps opponent has bought a Thornmail for 2100 gold with 100 armor because he is a tank he gains armor more than other champs as he levels. Maybe he has a passive armor or Damage Absorb ability too (who knows).

Either way when that Tankie dps gets to max level he has about 200 Armor and my dmg is doing half of what it should be (granted I get all my stacks on him from black cleaver) ((--Note: 100 MR or 100 AR does results in halfing your damage if you have no penetration)

This is what riot calls balanced.

But oh wait!!! from no where the wild Tankie DPS is Cho or Garen with their True dmg ult.

LoL no worries tanks don't do damage pffft am I right?

Morello and his team: "Lets nerf burst mages more. " wheeeeeeeeee