Subtotalen berekenen werkt niet!

<table id="items">
<tr>
<th>Aantal</th>
<th>Prijs</th>
<th>Subtotaal</th>
<th>BTW %</th>
<th>BTW</th>
<th>Totaal incl.</th>
</tr>
<tbody>
    <tr class="targetfields"> 
        <td><input type="text" name="quantity" id="quantity" class="quantity" /></td>
        <td><input type="text" name="rate" id="rate" class="rate" size="5" /></td>
        <td><input type="text" name="subtotal" id="subtotal" class="subtotal" readonly="readonly"/></td> 
        <td><input type="text" name="btw" id="btw" class="btw" size="5" /></td>
        <td><input type="text" name="subbtw" id="subbtw" class="subbtw" readonly="readonly"/></td>
        <td><input type="text" name="totaal" id="totaal" class="totaal" readonly="readonly"/></td> 
    </tr> 
    <tr class="targetfields"> 
        <td><input type="text" name="quantity" id="quantity" class="quantity" /></td>
        <td><input type="text" name="rate" id="rate" class="rate" size="5" /></td>
        <td><input type="text" name="subtotal" id="subtotal" class="subtotal" readonly="readonly"/></td> 
        <td><input type="text" name="btw" id="btw" class="btw" size="5" /></td>
        <td><input type="text" name="subbtw" id="subbtw" class="subbtw" readonly="readonly"/></td>
        <td><input type="text" name="totaal" id="totaal" class="totaal" readonly="readonly"/></td> 
    </tr> 
    <tr class="targetfields"> 
        <td><input type="text" name="quantity" id="quantity" class="quantity" /></td>
        <td><input type="text" name="rate" id="rate" class="rate" size="5" /></td>
        <td><input type="text" name="subtotal" id="subtotal" class="subtotal" readonly="readonly"/></td> 
        <td><input type="text" name="btw" id="btw" class="btw" size="5" /></td>
        <td><input type="text" name="subbtw" id="subbtw" class="subbtw" readonly="readonly"/></td>
        <td><input type="text" name="totaal" id="totaal" class="totaal" readonly="readonly"/></td> 
    </tr> 
    </tbody>
 <tfoot>
 <tr>
 	<td></td>
    <td></td>
    <td><input type="text" id="total"></td>
    <td></td>
    <td><input type="text" id="tbtw"></td>
    <td><input type="text" id="ttot"></td>
 </tr>
 </tfoot> 
</table>



<script>
$(function() {

    $("#items").keyup(function(event) {
     var total = 0;
     $("#items .targetfields").each(function() {
         var qty = parseInt($(this).find("#quantity").val());
         var rate = parseInt($(this).find("#rate").val());
		 var btw = parseInt($(this).find("#btw").val());
         var subtotal = qty * rate;
		 var subbtw = (qty * rate) / 100 * btw;
		 var totaal =  ((qty * rate) / 100 * btw) + (qty * rate);
         $(this).find("#subtotal").val(subtotal.toFixed(2));
         $(this).find("#subbtw").val(subbtw.toFixed(2));
         $(this).find("#totaal").val(totaal.toFixed(2));		 
		 		 
         if(!isNaN(subtotal))
             total+=subtotal;
     });
     $("#total").val(total.toFixed(2));
     $("#tbtw").val(tbtw.toFixed(2));
     $("#ttot").val(ttot.toFixed(2));
    });

})
</script>