A taxable bond with a coupon rate of $6\mathrm{.}00\%$ has a market price of $99\mathrm{.}42\%$ of par. The bond matures in $18\mathrm{.}00$ years ans pays semi$-$annually. Assume an investor has a $22\mathrm{.}00\%$ marginal tax rate. The investor would prefer otherwise identical tax$-$exempt bond if it's yield to maturity was more than $\_\_\_\_\_\%$