[j0hnik]

<select>
		<option value="0">0</option>
		<option value="10">10</option>
		<option value="20">20</option>
	</select>

	<select>
		<option value="0">0</option>
		<option value="10">10</option>
		<option value="20">20</option>
	</select>

	<output></output>

	<script>
		var sel = document.querySelectorAll('select');
		sel.forEach(el=>el.onchange = e => document.querySelector('output').value = [].reduce.call(sel, (c,el)=>c+el.value*1,0)); 
	</script>

вот вам пример