Handling Questions You Cannot Answer
What to do in the moment when you genuinely don't know the answer to a question — the difference between a response that ends the interview well and one that makes it worse.
Every interview eventually hits a question the candidate doesn't know cold. What separates a strong response from a weak one isn't whether you know the fact — it's what you do in the ten seconds after realizing you don't.
Say what you know, not that you know nothing
The weakest response is a flat "I don't know," delivered as a full stop. It gives the interviewer nothing to work with and ends the exchange. A much stronger response narrows the gap: "I don't remember the exact formula, but I know it involves the ratio of X to Y, and I know it should behave like Z in the limit — let me try to reconstruct it from there." This tells the interviewer you have a partial map of the territory even without the exact answer memorized, and it often lets you rebuild the missing piece live, which is more impressive than reciting it from memory would have been.
If reconstruction genuinely isn't possible — you've truly never seen the concept — say that plainly and pivot to the closest thing you do know: "I haven't studied that model directly, but here's how I'd reason about it from first principles" or "I don't know that one, but I do know a related result — is it okay if I talk through that instead?" This keeps you contributing rather than going silent, and it shows the interviewer how you handle a genuine gap in your knowledge, which is itself useful information for them.
Example
Asked to state the Black-Scholes formula from memory and drawing a blank, a strong answer is: "I can't recall the exact closed form right now, but I know the inputs are spot, strike, volatility, time to expiry and the risk-free rate, and I know the price increases with volatility and time to expiry. Do you want me to reason through the intuition instead of the formula?" That response salvages a genuinely useful few minutes of conversation out of a moment that would otherwise be a dead end.
When you don't know an answer, say precisely what you do and don't know instead of stopping at "I don't know" — partial knowledge, reconstructed from first principles or adjacent facts, is far more informative to an interviewer than silence, and it's often enough to get most of the way to the answer.
Never guess and present a fabricated answer with false confidence — interviewers ask enough follow-up questions that a bluff almost always unravels, and it damages trust in everything else you said, including the parts you actually knew well.
Related concepts
Practice in interviews
Further reading
- Crack, Heard on the Street (interview technique notes)