Reminds me of one where the person is told a bus contains X amount of people when it leaves the depot. When it stops, x amount get off, x get on, then when it next stops, x get off while x get on. Next stop no-one gets off, x get on. This goes on for about 8 or 9 stops and the person being asked the question is doing his/her mental arithmetic, getting an answer ready for the final question. Which is, "How many times did the bus stop?"