Why computed field is not getting displayed while creating content
Try adding the following JavaScript to your theme or in a module (so it's added on the page you are working on with the fields):

It's pretty rudimentary, but it works!  You might want to expand it to work on keypresses rather than .change() events.

Drupal.behaviors.jqFill = {
    attach: function(){
      $('#edit-field-half-und-0-value, #edit-field-full-und-0-value').change(function(){
        half = parseInt($('#edit-field-half-und-0-value').val());
        full = parseInt(+$('#edit-field-full-und-0-value').val());
        total = half + full;

        if(!isNaN(total)){
          $('#edit-field-total-und-0-value').val(total);
        }
      });
    }
  }


Note I got the field names from your other question here.