A similar situation is very common in case interviews, and I would suggest the same advice applies:
- Ask for a minute or two to think through the question. It may feel a little strange but I don't know a firm which will have an issue with you taking this time
- Secondly, the interviewer is not trying to trick you, so if you start suggesting some reasonable idea's your interviewer may well help steer you. They are not testing your ability to answer an individual question, but far more how you think. The best candidates don't always get the answer right first time
Hope that helps!