But that's not how problem solving works. You need work your way from what you do know to get to an answer for what you don't. It's not guessing -- it's taking the background knowledge you do have and applying it to the problem at hand (and every engineer should have some background knowledge, even if it's just basic arithmetic).

That's what these kinds of questions are testing. Someone who immediately gives up is probably not going to be resilient in the face of new challenges.