Just try, at most get it wrong, at best get some marks. The risk reward is so good, why not do it?
Sometimes, they will say it's too complex or they've never seen this kind of questions before. I think here's where perseverance comes in. If you don't own the problem, then it's so easy to say I don't know. If you give up, then the teacher will swoop in and help you out. It's so convenient. But if you really treat it as YOUR problem, you'll attempt again and again until the problem is solved. It's the lack of trying that irks me, not the failure to get the right answers.
Don't they say, being present is half the battle won? Lack of attempt is like being absent!
Do yourself a favor, do not give up so easily! Just write something sensible, instead of leaving it blank. If you write something, you might get some marks. If you leave it blank, you'll definitely get nothing!