jQuery -prevUntil() by Norlihazmey Ghazali

http://stackoverflow.com/questions/35715222/calculate-sum-of-each-td-having-similar-class/35715588#35715588

HTML

<table>
  <tr>
    <td class="20">1</td>
    <td class="20">2</td>
    <td class="20">3</td>
    <td class="20">4</td>
    <td class="20">55</td>
    <td class="20">3</td>
    <td class="total"></td>
    <td class="30">2</td>
    <td class="30">5</td>
    <td class="30">6</td>
    <td class="total"></td>
  </tr>
</table>

JavaScript

$('.total').each(function() {
  var sum = 0,
      elem = $(this).prevUntil('.total');
  elem.each(function() {
    sum += +$(this).text();
  });
  $(this).text('Total :' + sum);
});