confused about armor protection and natural protection

[Norfleet]

Some items, like the invulnerability spell, don't increase protection, they set it: So if you're affected by Invulnerability, Robe of Invulnerability, or maybe the Amon Hotep, anything which fails to increase your protection over 30 has no effect, and anything which increases your protection to or over 30 makes invulnerability irrelevant: They're not an armor effect, they're simply if prot < 30, prot = 30.

Here is the formula for you, from the Newbie FAQ.

How do you calculate total protection on a unit with natural protection (thick skin) and armor? According to Johan O.:
"Pb = base protection, intrinsic, from stone skin etc.
Pa = protection from armor
Pt = total protection
Pb and Pa are always restricted to 40 or less. (This quote has been changed because the forum Boards cannot handle greater than and less than symbols - Cherry)

Pt=Pb+Pa -(Pa*Pb)/40
So a unit with 10 intrinsic 10 protection and a total of 18 in equipment armor would have an effective armor of: 10+18-(10*18)/40 = 28-4.5 = 23.5, rounded up to 24"

[onomastikon]

Ok thanks. I am not good at arithmetic.