Armor Pen Calculations

So recently, i was trying to see how much armor penetration i could get on some champions and i have a couple of questions:

1. Does 1 armor pen reduce armor by 1 from the opponent? (if i have 30 flat armor pen, does it subtract 30 armor from the person i attack?)

2. How does the +45% bonus armor penetration work of the new last whisper item? (does it add 45% of your armor pen to your armor pen?, like if i have 50 armor pen, would it give me 22.5 armor pen?)

3. For the mastery +7% bonus armor penetration, does it work similarly to the new last whisper item?

4. If I have 30 armor pen and someone only has 20 armor, will i be doing true damage?

If anyone could answer these for me, that would be greatly appreciated :D