He ido Abansando Esto es lo que tengo hasta ahora
<html >
<head>
<title>Formulario Maestro Detalle</title>
<style type="text/css">
<!--
.Estilo3 {color: #000066; font-weight: bold; }
-->
</style>
</head>
<body>
<form name="fmd" method="post" action="">
<table width="100%" border="1">
<tr>
<td width="10%">Orden No.
<br />
</td>
<td width="90%"><p>
<input name="textfield" type="text" size="4" maxlength="4" />
<br />
</p> </td>
</tr>
<tr>
<td valign="top">Fecha:
</td>
<td width="90%"><input type="text" name="textfield2" /></td>
</tr>
<tr align="left" valign="middle" bordercolor="#0000FF">
<td colspan="2"><table width="100%" border="0" cellpadding="0" cellspacing="0">
<tr bgcolor="#CCCCCC">
<td align="center" ><span class="Estilo3">No.</span></td>
<td align="center" ><span class="Estilo3">Descripcion</span></td>
<td align="center" ><span class="Estilo3">Cantidad</span></td>
<td align="center" ><span class="Estilo3">Precio</span></td>
<td align="center" ><span class="Estilo3">Monto</span></td>
</tr>
<?php
for ($i=1; $i<=5;$i++){
echo "
<tr>
<td >$i</td>
<td><input type='text' name='desc$i' size=\"110\" > </td>
<td><input type='text' name='cant$i' onChange=\"calcula($i)\" size=\"6\"> </td>
<td><input type='text' name='pu$i' onChange=\"calcula($i)\" size=\"12\"> </td>
<td><input type='text' name='mont$i' readonly size=\"12\"> </td>
</tr>";
}?>
</table> </td>
</tr>
<tr>
<td colspan="2"><table width="31%" border="0" align="right" cellpadding="0" cellspacing="0">
<tr>
<td colspan="2" align="center"><input onClick="calcular()" name="b" type="button" value="Calcular"></td>
</tr>
<tr>
<td align="right"><strong>Sub Total </strong></td>
<td width="80" align="left" valign="middle">
<input name="subt" type="text" readonly > </td>
</tr>
<tr>
<td align="right"><strong>Descuento%</strong></td>
<td align="left" valign="middle"><label>
<input type="text" name="desc">
</label></td>
</tr>
<tr>
<td align="right"><strong>ITBIS</strong></td>
<td width="80" align="left" valign="middle">
<input name="itbis" type="text" readonly > </td>
</tr>
<tr>
<td align="right"><strong>Total</strong></td>
<td width="80" align="left" valign="middle">
<input name="total" type="text" readonly> </td>
</tr>
</table>
<label></label></td>
</tr>
</table>
</form>
<p> </p>
</body>
</html>
<script language="javascript1.2" type="text/javascript" >
function calcula(obj){
var monto ="mont"+obj;
var pu ="pu"+obj;
var cant ="cant"+obj;
var montot =0.00;
var precio =0.00;
var cantidad =0.00;
cantidad =parseFloat(document.fmd.elements(cant).value);
precio =parseFloat(document.fmd.elements(pu).value);
montot = cantidad * precio;
if (!isNaN(montot)){
document.fmd.elements(monto).value = parseFloat(montot);
}
calcular();
}
function calcular(){
var montot = 0.00;
var subtotal= 0.00;
for (i=1; i<=5; i++){
var nombre = "mont"+i;
montot = parseFloat(document.fmd.elements(nombre).value) ;
if (!isNaN(montot)) {
subtotal = montot + subtotal;
}
}
}
</script>