<table id="example">
       <thead>
          <tr>
             <th>Subject</th>
             <th>By</th>
             <th>Location</th>
          </tr>
       </thead>
       <tbody>
          <tr>
             <td>1</td>
             <td>2</td>
             <td>3</td>
          </tr>

// if row index = 6 then show advt image by merging columns. I gather colspan is not supported.

          <tr>
             <td colspan="3">show advt</td>
             <td style="display: none;"></td>
          </tr>
       </tbody>
    </table>

<td style="display: none;"></td>

$('tr:nth-child(6n)').after('<div>ad</div>');