Magic Penetration doubt

Comparing these 3 items: {{item:3135}} {{item:3020}} {{item:3151}} ... they all give Magic Penetration, however both Void Staff and Liandry's Torment have the magic penetration value in percentage so i'm not really sure the actual value of the Sorcerer's Boots, it says +15, is it +15%? EDIT: I made a mistake only Void Staff has the value in % sorry.
Report as:
Offensive Spam Harassment Incorrect Board