You would need some changes in the logic, and also use the dreaded eval
.. (if you want an agnostic way of handling the calculations)
$('#Selection').change(calc); //update result when changing the number
$("#number").keyup(calc); //update result when select changes
function calc() {
$('#result').val( eval( $('#number').val() + $("#Selection").val() ) );
}
and HTML
<div>
<input id="number" type="text" />
<select id="Selection" name="D1">
<option value="+1">Plus 1</option>
<option value="-1">Minus 1</option>
<option value="*2">double</option>
<option value="/2">half</option>
</select>
<input id="result" type="text" />
</div>