How to determine which textbook problems are the most useful when studying? If I have to study for an exam, like organic chemistry, I know that I should be studying my notes and, most importantly, practice problems.

However, when you have to review for final that is cumulative, you have to go through potentially hundreds of problems assigned. I understand the best case scenario is that you budget your time throughout the weeks to do them all, but some problems are just repeats of a very simple concept (sometimes with a nuanced variation).

My question is: how do you determine out of all the problems assigned which ones would be the most effective to study for the final/an exam?
Some books list problems that increase in difficulty, so do the first one, the last one and one or two from the middle...