calculating a mark

calculating a mark

I'm making a page that calculates a mark before I submit the data to the database. It involves 8 <select> pulldowns. Each time I add a number I'm trying to get this Jquery code to display a percentage. The first one works but when I add any subsequent mark the display div shows NaN. 

Any help would be appreciated.  Here is my Javascript.

  1. $('select').change(function() {
  2. var arr_mark = $('#arr_mark').val();
  3. var head_mark = $('#head_mark').val();
  4. var solo1_mark = $('#solo1_mark').val();
  5. var solo2_mark = $('#solo2_mark').val();
  6. var rhyth_mark = $('#rhyth_mark').val();
  7. var overall_mark = $('#overall_mark').val();
  8. var prep_mark = $('#prep_mark').val();
  9. var out_of;
  10. if($('#head_mark')=='NA'){
  11. out_of = 45;
  12. head_mark = 0;
  13. }
  14. else {
  15. out_of = 50;
  17. }
  18. var mark = ((arr_mark+head_mark+solo1_mark+solo2_mark+rhyth_mark+overall_mark+prep_mark)/out_of)*100;
  20. $('#display').html(mark+'%');
  22. });