javascript simple calculation between text box

Easy points for a javascript Guru. I know this is not what EE is for I should research a script and come up with the problems I have to implement it but I feel this is probably simple enough for a Guru to write it up from scratch rather than patch an existing script. If you guys feel differently let me know and I will come up with something

I need a simple calculation script add and substract between the fields

It is basically to perform maintenance in baseball card collection

There is 4 fields here is how they should interact

  Owned and Trade is the total of that card owned by the user so the number entered in Trade need to be substracted from Owned the reverse is also true
 
  A user can add more card in Owned after the Trade is down to zero i.e Owned 15 Trade 5 Total 20 cards if the user want to add a card to his inventory (to make it 21) adding a card to Owned should only move a card from Trade to Owned (owned 16 - trade 4) to increase the total to 21 it should be Owned 21 Trade 0

  Delete: the number of card entered in this box need to be substract from Trade and from Owned if there was not enough in the Trade column
 
  Wanted: stand alone, number can be edited without changing other fields
 
The best of the best would be to use AJAX to update the database as we go but I can live with a submit button to an update script.

The list is populated from a mysql database there is a small sample on how it is produced/displayed

The script
<?php
include 'CONNECT TO DB';
 
$sql = mysql_query("SELECT * FROM My_Cards WHERE user_id = '1' AND set_id = '262'");
 
         echo "<form action=\"update_mycards.php\" method=\"POST\">
               <table border=1 width=\"500\">
                <tr>
                   <th>Card ID:</th>
                   <th>Owned</th>
                   <th>Wanted</th>
                   <th>Trade</th>
                   <th>Delete</th>
                </tr>";
                              $i = 1;
      while($data = mysql_fetch_array($sql)){
        
              $query = mysql_query("SELECT card_title FROM Non_Sports WHERE card_id = '$data[card_id]'");
 
                    while($row = mysql_fetch_array($query)){
                      
                        $title = $row['card_title'];
                        $title = stripslashes(strip_tags($title));
 
           echo "<tr>
                    <td>$title</td>  <td><input type=\"text\" name=\"owned$i\" size=\"5\" value=\"$data[owned]\"></td>  <td><input type=\"text\" name=\"wanted$i\" size=\"5\"  value=\"$data[wanted]\"></td>  <td><input type=\"text\" name=\"trade$i\" size=\"5\" value=\"$data[trade]\"></td> <td><input type=\"text\" name=\"delete$i\" size=\"5\"></td>
                </tr>"; $i++;
      }
      }
           echo "</form>
                 </table>";
 
?>

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