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" />