[ksa]

Мои 5коп...

var a = [6, 7, 8, 14, 15, 16, 20, 21];
var b=[];
for (var i = 0; i < a.length; i+=3) {
	b[b.length]=a[i]+' - '+((a[i+2])? a[i+2]: a[i+1]);
};
alert(b);