[рони]

ksa,
где 6-9 ?

var a = [6, 7, 8, 9, 14, 15, 16, 20, 21];

var b=[];
for (var i = 0; i < a.length-1; i+=3) {
	b[b.length]=a[i]+' - '+((a[i+2])? a[i+2]: a[i+1]);
};
alert(b);