The obvious factor that I've mentioned before is that certain easy-to-understand principles simply seem to confuse Michael and probably always will. As we see here, he seems to be incapable of realizing that half of something will ALWAYS be half of something while all of something will always be all of something. It doesn't matter if you use the words "percent" or "half" and "all" to describe it, he seems to want to believe that there might be a case where half of something is NOT half of something just to prove that he isn't stupid and that math IS a boring waste of time he should never have been asked to do.
This, as I've also mentioned before, makes of him another iteration of Lynn's very real need not to be asked to do things that bore her lest she be proven wrong about how hard they really are and suffer the painful humiliation of having authority figures spent years making her feel like shit by ranting about how wrong she was about being painfully humiliated. As we see in these example of Mike wanting to not make the best of the bad situations his pissy attitude kept landing him in, our hero thinks that he shouldn't be asked to actually find out that math actually is easy lest he suffer the defeat of proving his asshole parents who never shut their ugly gobs when they win right. Simply put, Elly's overweening need to inform Michael that he isn't allowed to be right about anything because that's bad and wrong is why to this day, our boy has to take his shoes off to count to twenty.