I think being able to explain these answers is appropriate for children who are at that level and have to actually think about the problems. For those who are beyond that it is torturous and mind-numbing. As adults, we wouldn't need to explain how we know 2+2=4 and a kid who is past that shouldn't either. It's already completely obvious without having to put a bit of thought into it. If kids have already shown mastery of the concepts, they shouldn't even be doing those problems, much less explaining or showing work.
I would ask if my kid could test out of it. Of course, we all know how that goes with most teachers.
