<!DOCTYPE html>
<html>
<head>
<style>
p { color:blue; margin:8px; }
</style>
<script src="http://code.jquery.com/jquery-latest.js"></script>
</head>
<body>
<input type="text" id="s1" value=""/> * <input type="text" id="s2" value=""/> = <input type="text" id="s3" value=""/>
<script>
var value1=0;
var value2=0;
$("#s1").keyup(function(){
value1 = $(this).val();
$("#s3").val(value2*value1);
});
$("#s2").keyup(function(){
value2 = $(this).val();
$("#s3").val(value2*value1);
});
</script>
</body>
</html>
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