Except some professors / PhDs deliberately set up exercise in a way that they involve large-ish (3-5 digits) or stupidly large (9+ digits) numbers to turn otherwise trivial exercises mostly into a mental math test. (Another all-time favourite of mine was the guy who always included an exercise in his exams that required you to determine the non-trivial prime factorization of a four digit number).
Good exam questions can be approached in multiple ways and are not a mental math test.
Bad math exam questions are usually those that can only be solved using one specific technique or lemma (ideally one that was only mentioned in passing once or twice) or require a lot of error-prone calculation.
One clue you were going off course on a test in a high level math class was when all your numbers were getting rather crazy and unwieldy...
We had one professor who was rather infamous for being really sloppy when writing exam questions. On more than one occasion did he accidentally transpose a couple of numbers in a problem, turning an equation that should have easily reduced to a trivial linear problem into a 4th degree polynomial with complex roots.
Good exam questions can be approached in multiple ways and are not a mental math test.
Bad math exam questions are usually those that can only be solved using one specific technique or lemma (ideally one that was only mentioned in passing once or twice) or require a lot of error-prone calculation.