Work on the problems you care about and are interested in solving. The reasoning for that is that when you are both interested in and care about a problem you w

Work on the problems you care about and are interested in solving. The reasoning for that is that when you are both interested in and care about a problem you will give it all of your attention. When you work on a problem that you think "should" be worked on but that you don't deeply care about, then you won't give it all of your attention. That leads to less job satisfaction and solutions that aren't as good as those that would be come up with by someone who is really invested. The meta question is one that is more important, do I work on a problem that pays me a lot of cash or one that is important. Bill Gates showed you can earn a lot of cash and then work on important problems, but I don't think the odds favor that approach.