Oppure, senza premere pulsanti:
codice:
<script>
function percentuale() {
 a = document.form.a.value/100;
 b = a*document.form.b.value;
 document.form.total.value = b
 }
</script>
<form name="form">
   <input name="b" onkeyup="percentuale()"> 
   <select name="a" onchange="percentuale()">
       <option value="20">20%</option>
       <option value="30">30%</option>
       <option value="40">40%</option>
   </select>
   <input name="total">
</form>