Please search forums before posting any new question/thread. Also read, http://docs.oracle.com/cd/E37097_01/doc/doc.42/e35122.pdf before posting basic questions.
As Denes correctly said, tons of examples are there so no need of new thread needed.
-- Sunil Bhatia
Just creating a page doesn't do anything. You have to create a page and then define a region. This region should be a internactive region preferably. If you created a interactive report then you can create a computation on runtime. Once you have done this you can save this report and make it the default report. Personally I would do these kind of computations on the database itself so the interactive report only displays the values that it fetched.