A Treasury bond with a par value of $100 and a price of $98.25 matures in 18 months. The bond has a coupon of 6.20% and pays interest semiannually with the next due in six months. Assume the zero rates for maturity of 6 months and 12 months are 5.20% and 6.45% (with continuous compounding). What is the zero rate (with continuous compounding) for maturity of 18 months?