Our school use to do the 'perms and combs' unit last but we are trying something different this year (our 'perms and combs' unit scored slightly below the rest of the units).  We are doing it 2nd last.  Hopefully, students will be able to focus more on this 'new' concept and we decided to leave 'Rational Functions and Function Operations' to the end.  We hope this unit is more of a review topic to the entire course rather than a new one.