#2 relies on understanding how numbers "work", i.e. on having spent time on playing with them. #5a is a factorization exercise. In general, Russian approach t

#2 relies on understanding how numbers "work", i.e. on having spent time on playing with them. #5a is a factorization exercise. In general, Russian approach to the math includes exposing kids to the toolset as well as the theory. This particular case is solved this way, that is solved that way, etc. Keep on it for a while and it tends to develop an intuition for knowing right away which problem is solved with which approach. So, yeah, if there are shortcuts, you use them. If there aren't any, you brute-force it.