onKeyUp="javascript:document.form.textfield.value = parseInt(document.form.text1.value)

<form name="form" method="post" id="form1" name="form1">
<table border="1">
          <tr>
    <?php 
              for($i=0;$i<=4;$i+=2)
                   {
		?>
    	<td><input type="text" name="text<?=$i?>"></td>

<td><input type="text" name="text<?=$i+1?>" onKeyUp="document.form.textfield<?=$i?>.value=parseInt(document.form.text<?=$i?>.value)+parseInt(document.form.text<?=$i+1?>.value);"></td>

<td><input type="text" name="textfield<?=$i?>" id="numtxt"></td>
</tr>
<?php } ?>
</table>
</form>