"rule av 70" is simpel wae tu ges how long flate rate kut value bi half... rule sae if flate=x%/yeer, num yeers for value=half is 70/x... zampel: flate=4%/yeer...70/4=17.5 yeers tu luze 50%... But, 68 is kloeser...68/4=17 yeers... reel anser=16.9797 yeers (yuze exponenshals)