<input type="text" name="vtb2" id="vtb2" size="1" maxlength="3" value="640">

<input type="text" name="vtb3" id="vtb3" size="1" maxlength="3" value="540" readonly="true">
&nbsp;
<select id="flv" onchange="checkforma();">
<option value="43" selected>4:3</option>
<option value="169">16:9</option>
</select>

<script type="text/javascript">	
checkforma = function() {
if(document.getElementById('flv').value=='43'){
document.getElementById('vtb3').value=Math.round(document.getElementById('vtb2').value/4)*3;
}
if(document.getElementById('flv').value=='169'){
document.getElementById('vtb3').value=Math.round(document.getElementById('vtb2').value/16)*9;
}
}
checkforma
document.onkeyup = checkforma
</script>

<input type="text" name="vtb2" id="vtb2" size="1" maxlength="3" value="640" onkeyup="fn()">
 
<input type="text" name="vtb3" id="vtb3" size="1" maxlength="3" value="540" readonly="true">


 
<script>  
function fn(){
	inputs=document.getElementsByTagName('input');
	inputs[1].value=parseInt(inputs[0].value)-100
	
	}


</script>

<input type="text" name="vtb2" id="vtb2" size="4" maxlength="3" value="640" oninput="fn(this)">
 
<input type="text" name="vtb3" id="vtb3" size="4" maxlength="3" value="540" readonly="true">


 
<script>  
function fn(a){
	input=document.getElementById('vtb3');
	var b = parseInt(a.value);
 	    if(!isNaN(b)){
	      input.value=parseInt(b)-100;
	    }
}


</script>