A smart colleague gives a quick answer to a numerical puzzle that seems plausible. What's the most useful next step?

Answer & explanation

Check the constraints with a simple calculation

An answer that arrives quickly and confidently may reflect a fluent intuition, not a careful analysis. When consequences matter, identify the precise condition that must hold and test the answer against it, perhaps by calculating a counterexample. This selectively activates deliberate reasoning without requiring you to second-guess every routine thought.