I like to think that Hank deals with brain damage from the copious head injuries that he receives on the daily (exaggeration). A lot of the mental effects (bad attention span, difficulty solving problems or understanding abstract concepts, etc.) stay around longer than the physical ones (difficulty speaking, tremors, bad coordination, etc.), SO Doc will give him a bunch of little mental exercises like math stuff and word problems because he HATES FUN. Hank understandably doesn't like this and doesn't make an effort to do them. Untillllll Doc recontextualize it to gun stuff for Hank;
"Here's five different guns, each with a different capacity magazine. Tell me how many rounds there are."
"If you fired [x gun] and [y gun] seven times each, and then [z gun] four times, how may rounds do you have left overall?"
"Imagine I gave you two magazines, one for [a gun] and one for [b gun]. Now how many rounds do you have left?"
and so on. It ends up working better than Doc imagined (which means it actually worked at all), but a win is a win.