var hoverzones = $$("#diagram div.hoverzone");
$each(hoverzones, function(div, index) {
	var	zone = Math.floor(index/3) + 1;
	div.addEvent("mouseenter", function() { $('bdiv'+zone).addClass('over'); });
	div.addEvent("mouseleave", function() { $('bdiv'+zone).removeClass('over'); });
	div.addEvent("click", function() { make_active(zone) } );
});

var make_active = function(zone) {
	for(var x = 1; x<=5; x++) {
			if(zone != x) {
				$('bdiv'+x).removeClass('active');
				$('cdiv'+x).removeClass('active');
			}else{
				$('bdiv'+x).addClass('active');
				$('cdiv'+x).addClass('active');
			}
	}	
}

make_active(1);<!-- 
 
 -->