Georgie,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
div{
background-color: #FF00FF;
height: 70px;
width: 200px;
margin: 5px;
}
fieldset:hover div{
opacity: .2;
}
fieldset:hover div:hover{
opacity: 1;
}
fieldset{
width: 200px;
}
</style>
</head>
<body>
<fieldset>
<div id = 'one'></div>
<div id = 'two'></div>
<div id = 'three'></div>
<div id = 'four'></div>
</fieldset>
</body>
</html>