Sorun çözülmüştür 
<script type="text/javascript">
function hesaplama()
{
var sayi1 = document.getElementById('sayi1').value;
var sayi2 = document.getElementById('sayi2').value;
if(sayi1=="" || sayi2==""){
document.getElementById('sonuc').value = "";
}else{
document.getElementById('sonuc').value = parseInt(sayi1) * parseInt(sayi2);
}
}
setInterval("hesaplama();",200);
</script>
<title></title>
</head>
<body>
<select id="sayi1">
<option value="1">1</option>
<option value="2">2</option>
</select>
<select id="sayi2">
<option value="1">1</option>
<option value="2">2</option>
</select>
Sonuç : <input type="text" id="sonuc" />