Talk:Xerath/Strategy/@comment-209.152.45.35-20111006232523/@comment-70.176.221.190-20111007040309

You must have had some other source of mpen then, because 30% pen plus 40% pen results in 58% mpen. The math on it would be .7 (damage after 30% pen) x .6 (damage after 40% pen), which equals 42%, the percentage of damage that ends up being dealt. The inverse of that gives you 58% magic penetration