how to calculat two input box and get resukt in third input box by javascript?

<code><table><tr> 

 <?php

if($_POST['class'])

{

$sql6=mysql_query("select * from sr_reg where present_class='$_POST[class]' order by id ASC");

$num6=mysql_num_rows($sql6);

if($num6)

{

$i=0;

while($val6=mysql_fetch_array($sql6))

{

$i=$i+1;

?>

 

<form name="frm1" action="class.php" method="post">

  <td  bgcolor="#FFFFFF"><div align="center"><strong>

  <?php echo $i?><input type="hidden" name="class" value="<?php echo $_POST['class']; ?>" />

</strong></div></td>

     <td bgcolor="#FCD5B4">

   <div align="center"><input type="text" name="roll" value=""  /></div></td>

<td bgcolor="#C5BE97">

   <div align="center"><?php echo $val6['studentname']." ".$val6['sname'];?></div></td>

     <td bgcolor="#D99795"><input type="text" name="oral"   id="txt_oral[]"  onkeyup="totalMarks();"/></td>

         <td bgcolor="#8DB4E3"><div align="center"><input type="text" name="writen" id="txt_writen[]" onkeyup="totalMarks();" /></div></td>

        <td bgcolor="#D7E4BC" colspan="2"><div align="center"><input type="text" name="tot"  id="txt_tot[]" /></div></td>

     

      

</form></tr>

<?php

 

}

 }

else

{}

}

else

{

 

}

 

?>

 </code>