Are math questions copyrightable? I am thinking it is, but to what extent and what are the limitations? Because some question like 2+2 are too basic to be copyrightable, but some other mathematics question in calculus 1, 2 and 3 are easily copyrightable, or at least should or might have to be. I am asking, because some people may copy other people's questions and put them in their text or textbook.
At some point, a word problem that expresses an underlying mathematical problem that could be posed in many different ways, could be subject to copyright. But some word problems that are effectively unique (e.g. derive Kepler's laws from Newton's law of gravity) could probably not be copyrighted, and others would be, as you note, too basic to have any creative or original element to them.
Likewise, stylistic aspects of the presentation that aren't necessary by mathematical convention (e.g. presenting all advanced questions in boxes with borders in large print blue type) could be entitled to copyright protection.
Further, merely having identical questions doesn't prove a copyright violation. You have to prove in an infringement action that one question was derived from the other question (e.g. by showing that the alleged infringer had the textbook allegedly copied and carried over typos into their edition). Usually, showing derivation is trivial, but in a case like this one, it might be hard to show.