javascript_pupil,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
</head>
<body>
<form action="http://" name="test">
<input name="bumbum" type="radio" value="5">
<input name="bumbum" type="radio" value="15" checked="checked">
<input name="field_name"><input name="" onChange="alert([this.value, document.test.field_name.value, document.querySelector('[name=\'bumbum\']:checked').value])" >
</form>
</body>
</html>