calculate form fields using jquery-Collection of common programming errors

You can bind a change() to both the :radio and elements to calculate the total.

$("select[name='number'], :radio[name='type']").change(function(){
    var $r = $(":radio[name='type']:checked");
    var $d = $("select[name='number']");
    if($r.length ==1){
        var t = parseInt($r.val(), 10);
        var n = parseInt($d.val(), 10);
        $("#result").val(t*n);
    }
});

Example on jsfiddle