6cfn
That's exactly what you do:
"I'm not sure but because I know [something] and [something] I would think that the answer is probably this..."
You're not expected to know everything. Some people in this thread are making it seem like if you don't know every engine model etc... by heart you will fail the interview. That simply isn't true. Yes, you should have a good idea about these sorts of things but far more important than memorizing facts is the ability to think through a problem and come up with a logical solution. If you can do that it will go a lot further with the interviewer than simply memorizing things.