Armor penetration

Can any1 explain me damage?
Let's look it like this:

You deal 20 damage( with sword),how many against:

armor 60:
armor 70:
armor 80:

You deal 20 damage (with sword) + attack skill (with +20 damage):

armor 60:
armor 70:
armor 80:

You deal 20 damage (with sword) + attack skill with +20% armor penetration (that deal +20 damage):

armor 60:
armor 70:
armor 80:

I need help to find out damage calculation(but don't give me url from guildwiki because I don't understand that from there)

20 w/ no penetration = 20 vs. 60, 16.82 vs 70 and 14.14 vs 80.

The +damage is armour-ignoring and simply a damage shift on the equation = 40 / 36.82 / 34.14

With 20% armour penetration the attack deals = 24.62 / 21.44 / 18.66 damage, plus the 20 from the attack skill (+dmg is not affected by penetration).

So... In case you were wondering, Strength and Sundering are kinda lame.

The calculations can be found out with hte calculator here on Guru, there's a link on the left side on the main page.

Wild Blow and Decapitate can help with those tests, or you can try this out:

http://www.guildwarsguru.com/calculators/guru_armor.php

Don't forget about the wooden targets outside Balth's temple too.