I struggle with the math part too, but not because of the math.
The mechanic is actually pretty hard to follow. First time (going blind) I didn't even know our HPs were supposed to be used in the equation. That's how unclear the mechanic was from the start. They should maybe call attention to that more.
The second part of it, is in the way the question is asked. It is a convoluted way of basically saying "add this" or "subtract this." Having to think subtract/add when he's telling you to divide... I can see that being extremely hard for some people. Division/multiplication is probably the hardest part of basic math that people struggle with.
The third part of it, is that it's all in flying text, which a lot of the times I completely ignore. Usually because it's irrelevant jibberjabber from NPCs. The game has kind of trained me to ignore that stuff.
Add (sorry) to all of that, someone's difficulty with math, and you end up with one confusing round of "what the hell am I doing?"
That being said, I've only run the raid twice. So I don't think they should remove the mechanic. But maybe next time they can think on ways to make it easier to follow. And possibly this go around, make the penalty less severe on failure.