It was a theoretical example (hence why I wrote between brackets 'non wow related). I'm not a math expert either btw. But the question is; what is the chance that the next attack can be parried, dodged or resisted i.e. what is the chance that one of the following events occur (where parry, dodge & resist are the events). Computing the chance that multiple events occur at the same time or computing the total chance to avoid the next attack is a different thing.
Connect With Us