How exactly does armor piercing work?

[Droll]

Since there are now weapons with upto 30% in penetration bonus/penalty I was wondering how damage reduction of armor was calculated.

Say that opponent's armor reduces cut damage by 40% but you are using a hacker with 30% penetration bonus.
Would the armor only reduce 10% of the damage, or would it reduce 28% of the damage (30% of 40 is 12)
Likewise if your weapon had a 30% penalty to pen instead, would 70% of the damage get reduced or 52%?

Thanks