lo logre ¡¡¡¡
<script>
function CalcularMonto(){
var peso = document.form1.peso.value;
var precio = document.form1.precio.value;
var monto = peso*precio;
document.form1.subt.value = monto.toFixed(2);
//document.getElementById('subt').value=monto.toFixe d(2);
var iva = monto*19/100;
var total = monto+iva;
document.form1.subt1.value = monto.toFixed(2);
document.form1.iva.value = iva;
document.form1.total.value = total;
}
</script>
</head>
<body>
<form id="form1" name="form1" method="post" action="" onkeyup="CalcularMonto()">
<p>peso:
<input name="peso" type="text" id="peso" />
precio:
<input name="precio" type="text" id="precio" />
subtotal:
<input name="subt" type="text" id="subt" />
</p>
<p> </p>
<p> </p>
<p> </p>
<p> </p>
<p>............................................... ...............................
..............subtotal:
<input name="subt1" type="text" id="subt1" /><br />
.................................................. .................................................i gv:
<input name="iva" type="text" id="iva" />
<br />
.................................................. ...............................................tot al:
<input name="total" type="text" id="tot
al" />
</p>
</form>
gracias ¡¡¡¡