<script language="javascript">
function Calcola() { t = document.Perc.t.value; d = document.Perc.d.value; document.Perc.p.value = 100/t*d }
</script>

<center><b>
Metti <i>Totale</i> e <i>Dato</i>. Clicca e ottieni <i>Percentuale</i>
<form name="Perc">
Totale =      <input type="text" name="t" value="" size=10>  
Dato =        <input type="text" name="d" value="" size=30> <br/>
Percentuale = <input type="text" name="p" value="" size=30> (%)<br/>
<input type="button" value="clicca" onClick="Calcola()">
</form>
</b></center>