Armor Pen...Does the game calculate the percent or the flat first?

I was just curious as to whether the game applied flat armor penetration or percentage armor penetration first. Say a champion has exactly 100 armor. Say I have 85 percent armor penetration as with plus my passive e armor pen. Then...I have 15 flat armor pen from whatever it may be....say it is from runes. Does the game calculate the percent first? If so, I penetrate 85 percent of the 100 armor....leaving me with 15....which my flat armor pen finishes, and lets me do true damage. But.....if the flat armor pen is calculated first.....100-15 is 85, and 85 percent of 85 is is 72.25, and 85 subtract that 72.25 is 12.75, which the game would probably round to 13. In the end here, I'm not doing true damage.