Correct answers for those less mathematically inclined:
1. 15 is indeed 3x5, but she's supposed to factor the non-x (56) into (7x8).
2. She correctly factored (16) into (2x8), but it was supposed to be (4x4), and she mistakenly carried it over to the x.
3. She misread x²-5² as x²-(-5)², when it should have been x²-(5)².
You have to respect their dedication to one another. They're willing to drop everything to tutor her so she can stay in school and get an education, even if they're all dumbasses.
Ok, a little side-questing never hurt any manga, as long as it sets up a nice payoff later.
But, Jesus Christ, Naoya, it's just studying. What's with the screaming? I can feel my eardrums hurting from just reading your lines.
I... have concerns about this girl's survival if left alone. I mean it's clear that she's got some kind of attention disorder and hyper focus problems. In addition she's not book smart. And she's a little too flexible. And she doesn't have any confidence in herself. She can survive in the outside world because she's got street smarts and some common sense at least but if she fell in love with someone other than Naoya I have the feeling she'd meet a terrible fate