<html>
<script type="text/javascript">
function f() {
var el, i = 0, result = 0;
while( el = document.getElementById( "num_" + ++i ) )
result += +el.textContent;
document.getElementById( "result" ).innerHTML = "sum=" + result;
}
</script>
<div id="num_1">10</div>
<div id="num_2">20</div>
<div id="num_3">50</div>
<div id="num_4">30</div>
<div id="num_5">25</div>
<div id="num_6">64</div>
<input type="button" value="calculate" onclick="f();">
<div id="result"></div>
</html>