I was told something similar by my high school history teacher Mr. Kolhaise, but I think it was phrased in a more helpful manner and it's always stuck with me.

He said "Don't just tell me what you don't know, tell me what you do know" or in other words, don't just say "I don't know" but say something like "I don't know, but based on what I do know this is what I think the answer would be and this is how I would figure it out".

It really helps that everyone forgets what people say before the "but" and you can help get across the idea that while you may not know everything you know how to figure things out.