Graham purchased a business by agreeing to make three payments of $12 000 each in one year, two years, and four years. Because of cash flow difficulties in the first year, he renegotiated the payment schedule so that he would pay $16 000 in 18 months, $10 000 in 30 months, and a third payment of $10 000. In how many years should he make the third payment if interest is 7.2% compounded semi-annually?