PHP If Post Value is less than or greater than

I have a php booking form which checks to make sure that information has been entered in the boxes before its submitted:

I use the following to check to make sure group has a value in it.

if(!isset($_POST['group']) || trim($_POST['group']) == '')
{
    $error.= "Group Size is required<br/>";
}

How can I use an IF statement too check that the value in group is greater than 39 but less than 400?