If you have no idea about the answer to the question posed, then you should just move on. However, if you make a valid attempt, and get the answer wrong, then it probably makes sense to ask for the correct answer. You never know if you will be asked the same question again in a later interview so you might as well get it right the second time. The only time that I wouldn’t ask the interviewer for the answer is if you think the interviewer doesn’t know the correct answer (yes, bankers sometimes ask questions to which they don’t know the answer). If the interviewer doesn’t know, then he or she is just going to look stupid, and that’s never going to help your cause.