<input id=i1 value=30 onchange=f()><input id=i2 value=3 onchange=f()><input id=i3 value=90 ><br>
<script>
function f(){
var n1=document.getElementById('i1').value;
var n2=document.getElementById('i2').value;
document.getElementById('i3').value=Number(n1)*Number(n2);
}
</script>