My personal favourite is known as the tank problem. It can be phrased as such:You are working for the intelligence services in World War 2. A number of German tanks have been captured – with serial numbers 19, 40, 42 and 60. Assuming the numbers are assigned uniformly (no double numbering, no missed numbers) – how many tanks do the Germans have?

https://www.quora.com/What-are-some-of-the-coolest-math-problems-ever