можно и без кнопки
<html>
<head>
</head>
<body>
<table>
<tr> <td> type number: </td> <td> type percent: </td> <td> result </td> </tr>
<tr> <td> <input id="number" /> </td> <td> <input id="percent" /></td> <td> <input id="result" > </td> </tr>
</table>
<script>
get=document.querySelector.bind(document)
number=get("#number")
percent=get("#percent")
result=get("#result")
getPercent=function(){
if((number.value==="")||(percent.value==="")) return result.value="waiting..."
result.value=percent.value*(number.value/100)
}
number.oninput=getPercent
percent.oninput=getPercent
</script>
</body>
</html>