<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>
<input type="text" id="text1"> <br />
<input type="text" id="text2"> <br />
<input type="button" id="btn"> <br />
<input type="text" id="text3"> <br />
<script>
$("#btn").click(function() {
$("#text3").val(parseInt($("#text1").val()) + parseInt($("#text2").val()));
});
</script>