Getting the right answer on known problems is a prerequisite to getting the right answer on unknown ones. At least, if you can't get the right answer it's very unlikely you can solve ones where the answer is unknown. Also, there's a good chance you didn't really learn the material. Having taught courses at all levels I would say it's relatively rare to assign problems that are truly hard. Most just are testing your understanding of definitions and if you can't get the right answer you probably didn't understand something.
Getting a PhD isn't just about "learning" but also how to create/discover new knowledge. If all you want is to learn something get a second undergraduate degree or read on your own.
Getting a PhD isn't just about "learning" but also how to create/discover new knowledge. If all you want is to learn something get a second undergraduate degree or read on your own.