Currently, a three$-$year Treasury note pays $6\%.$ Assuming that your federal income tax rate is $20\%$ and state income tax rate is $15\%,$ what is the minimum interest rate that you would you need to earn on a tax$-$free municipal bond in order to buy it instead?

$1\mathrm{.}2\%$

$3\mathrm{.}9\%$

$4\mathrm{.}8\%$

$6\mathrm{.}35\%$