I want to select random questions from a Table from each catogories

SQL Server 2005 query needed, please help.

I have a table which have questions and all questions are categorized TopicWise wise.

And another table which Contains topic and its weightage of number in the Subject

Now, I need to select 50 random questions from Table QUESTIONS which are based on the weightage of the Topic like out of 50 random selected questions there must be

5 questions from Subtopic_Id=1 and    
15 questions from Subtopic_Id=2 and
10 questions from Subtopic_Id=3 and
10 questions from Subtopic_Id=4 and
10 questions from Subtopic_Id=5

See Second image (Table Topics)

and the number of topics and their weightage may vary in different subjects, so union statement will be a bad idea for a dynamic query.

I even have not figure out a single bit of line of code for this selection.