How To Convert Working Dates into Days

How to Convert Only Working Dates into Days.

SELECT START_DATE,END_DATE,
  (TRUNC(END_DATE) - TRUNC(START_DATE) ) +1,
  trunc(end_date,'d')
  /*((((TRUNC(END_DATE,'D'))-(TRUNC(START_DATE,'D')))/7)*2) -
  (CASE WHEN TO_CHAR(START_DATE,'DY','nls_date_language=english')='SUN' THEN 1 ELSE 0 END) -
  (CASE WHEN TO_CHAR(END_DATE,'DY','nls_date_language=english')='SAT' THEN 1 ELSE 0 END) as BusinessDays*/  abc
FROM table_name
where to_char(start_date,'mon-yyyy')='dec-2014';