Do they know it's a warmup and how much time you expect them to spend on it? I can easily imagine doing the question methodically and talking through my approach, and spending 20 min to do so. And I can also see the value in a candidate that approaches it that way, vs one who fires off the answer without doing some planning, even if it's an easy question.

I dont have context to know how you're asking it, but if you're not already, be clear about what you're looking for in the answer and the amount of time / effort you think the candidate should spend.