Some Interesting Requirements / Questions

Over the time I received few interesting requirements and questions. Few of them are straight forward that you do it in minutes, and few other questions are strange enough that some time have to sit and etch out the solution which is not top of the head. It would be only fair to share the information here. I would give you the requirement / question, if you can answer them, please add in comments.

You can use any language you like (except 3rd requirement). I was forced to use PL/SQL because of the project requirements.