how do you do dat math?!?

[quote=fifolek]so I see every now a thread where you do maths about how much dmg this does or how much dmg will be reduced to this and I don't get it as an example I am right now thinking, as a jungler I have full red runes armor penetration (15 ArmPen). if i now use the offensive masteries they give me 16% arm pen, how many arm pen reds could i switch for atack speed or something else if I want to have 15 arm pen? or another thing is, how can I calculate if the defense tree or offense tree is better for jungling gp/xin? yeah I could look up an existing build here or simply try it out, but yeah.. :/[/quote]

[quote=DEWO][url]http://leagueoflegends.wikia.com/wiki/League_of_Legends_Wiki[/url] + Common sense.[/quote]