In my experience from both sides of the interview, the most important thing in answering is the thought process for coming up with an answer. Make sure you have a firm understanding of the problem. You may need to ask for more details. Work through the problem out loud (silence does not help the interviewer). Declare an answer. Be prepared for followup/devils advocate questions that challenge your answer. Understand that it is not an insult, just questions to test your understanding of the problem, it's variables, and the solution you propose. If you don't know, don't be afraid to state that, but still work through the problem in earnest if the interviewer would like you to try. Generally giving up in an interview is bad, but the only thing that is worse is to spew out incorrect BS answers while pretending to know what you're talking about.