How to extract week number in sql

Question

I have a transdate column of varchar2 type which has the following entrees

01/02/2012
01/03/2012

etc.

I converted it in to date fo

Answer 1

Select last_name, round (sysdate-hire_date)/7,0) as tuner 
  from employees
  Where department_id = 90 
  order by last_name;