Hola otra vez:
Prueba este código:
Código PHP:
<html>
<head>
<title>sumar</title>
<script language="JavaScript">
var suma = 0;
Number.prototype.decimal = function(n) {
pot = Math.pow(10, parseInt(n));
return parseInt(this * pot) / pot;
}
function sumar(){
suma=parseFloat(document.form1.select1.value) + parseFloat(document.form1.select2.value);
document.form1.textfield.value=suma.decimal(2);
//document.getElementById('T1').value=(document.getElementById('textfield').value)*1.16;
}
function impuestos() {
document.form1.T1.value = (suma * document.form1.select3.value).decimal(2);
}
</script>
</head>
<body> <form name="form1" method="post" action="">
<p><b>TELARAÑAS A DOMICILIO ( PAGINA DE PRUEBA)</b></p>
<p>
<select name="select1" onChange="sumar()">
<option value="0" selected>ALTO cm</option>
<option value="17.21">100</option>
<option value="18.67">110</option>
<option value="20.13">120</option>
<option value="21.59">130</option>
<option value="23.05">140</option>
<option value="24.51">150</option>
<option value="25.97">160</option>
</select>
<select name="select2" onChange="sumar()">
<option selected value="0">ANCHO cm</option>
<option value="12.79">100</option>
<option value="14.05">110</option>
<option value="15.31">120</option>
<option value="16.57">130</option>
<option value="17.83">140</option>
<option value="19.09">150</option>
<option value="20.35">160</option>
</select>
<input type="text" name="textfield" size="20"> <select size="1" name="select3" onchange=impuestos()>
<option selected value="0">IMPUESTOS</option>
<option value="1">IVA EXENTO</option>
<option value="1.07">IMPUESTO IVA 7%</option>
<option value="1.16">IMPUESTO IVA 16%</option>
</select><input type="text" name="T1" size="20"></p>
</form>
</body>
</html>
Copié el de tu enlace y lo retoqué un poquillo...
Saludos
