Ranking and Filtering Based upon Prompt Value

I am working on a report in Web Intelligence that ranks a list of values in a list. Then a prompt value would be applied, be it 5, 10, 25, etc., that will filter the rank based upon the chosen number. So, for example, one of the prompt values is 20, so the report should filter to only show the top 20 records based upon a rank that was derived from a variable when 20 is chosen in the prompt. How can this be accomplished?