Talk:Magic penetration/@comment-187.127.147.95-20111114164123/@comment-71.212.144.229-20111213235844

"It would go down to 0. Penetration can't make MR go below 0." Not true. It says in this very article: "If a target has negative magic resistance, the amount of bonus damage is equal to the amount of damage reduction caused by that positive amount of magic resist. This sounds confusing and is better illustrated with an example. If a target has 10 magic resistance, they take 9.1% reduced damage (formula from magic resistance). Therefore a target with −10 magic resistance would take 9.1% bonus damage."

So actually, if your penetration is higher than their resistance, the target will have negative magic resist equal to the difference. So in your example, your target would have -30 magic resist (10-40=-30), and you would deal 27.3% bonus damage