An investor trading in bonds is looking for 25 % return on his purchases. He buys bonds with a few years left before maturity, holds them, and redeems them for face value when they mature. Looking through the bond listings, he finds a bond listed with a exist10,000 face value, that pays 12% interest on its face value each year, and is priced at exist8, 128. How many years (at most) must be left before maturity on this bond in order to meet the investor's desired percent return? (Enter your answer as a number rounded to one decimal place, without the percent sign.)