Usenet Newsgroups : rec.puzzles Hall of FameThe rec.puzzles Hall of Fame is a compilation of over 500 of the most popular puzzles that have been posted and discussed in the rec.puzzles newsgroup. In most cases a detailed solution has been provided. Many of these puzzles also appear in Braingle's own collection. 

You have an empty urn, and an infinite number of labeled balls. Each has a number written on it corresponding to when it will go in. At a minute to the hour, you take the first ten balls and put them in the urn, and remove the last ball. At the next half interval, you put in the next ten balls, and remove ball number 20. At the next half interval, you put in ten more balls and remove ball 30. This continues for the whole minute.... how many balls are in the urn at this point? (infinite) You have the same urn, and the same set of balls. This time, you put in 10 balls and remove ball number 1. Then you put in another ten balls and remove ball number 2. Then you put in another ten balls and remove ball number 3. After the minute is over, how many balls are left in the urn now? (zero) Are the above answers correct, and why or why not?
Solution