Identify an element without class or ID

Identify an element without class or ID

I am a total novice at javascript and jquery. I can sometimes understand the code I see, but have no idea how to write it. I have a SharePoint page that I wish to modify two specific <td> elements without modifying the master page. There is a <table> with a class that contains these two <td> tags. Please see the relevant code below. I am attempting to change the width value of each of these tags. Any help would be most appreciated.
  1. <table width="100%" class="ms-welcomepageheader" >
  2.       <trbody>
  3.             <tr>
  4.                   <td valign=""top" width="80%">.......</td>
  5.                   <td valign=""top" width="20%">.......</td>
  6.             </tr>
  7.       </trbody>
  8. </table>
  9.