People seem to be so terrified to utter the words "I don't know" when it comes to business. I once saw a public relations graduate student get on camera for a mock crisis communications interview. The second a "hard" question got asked, she freaked out and forgot her message and started making things up! Not good. Not only was the PR message not delivered, but the student had made all kinds of crazy, unreliable and untrue claims in the process. Saying "I don't know" is fine in the business world -- it shows you're not a know-it-all and proves you're not a liar. The trick to getting away with it is saying "I'm not sure, but I'd be happy to find out and get back to you," and then DO IT! Do the research, ask someone who knows, and get back to the person who asked you the question. Willingness to learn and help is never a bad thing.
