codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <title>Pagina senza titolo</title>
<script language="javascript" type="text/javascript">
// <!CDATA[

function calcola() 
{
    var s = document.getElementById('s').value; s = ((isNaN(parseFloat(s)))?0:parseFloat(s));
    var p = document.getElementById('p').value; p = ((isNaN(parseFloat(p)))?0:parseFloat(p));
    var g = document.getElementById('g').value; g = ((isNaN(parseFloat(g)))?0:parseFloat(g));
    
    var valore_campo_calcolato = 2*(s + p)*0.05 * g;
    document.getElementById("div1").innerHTML = valore_campo_calcolato;
}

// ]]>
</script>
</head>
<body>
    <input id="s" type="text" onchange="calcola();" />

    <input id="p" type="text" onchange="calcola();" />

    <input id="g" type="text" onchange="calcola();" />

    

    <div id="div1"></div>
</body>
</html>