Income from municipal bonds generally is exempt from federal, and sometimes state, income tax. For an investor with a marginal combined state and federal tax rate of $35\%,$ a taxable return of $6\%$ would yield only of $6\%.$ At that tax rate, a tax$-$exempt return of $3\mathrm{.}9\%$ is equivalent to a taxable return of $6\%.$ Find the tax$-$exempt rate of return that is equivalent to the given taxable rate of return for Sean.

$\backslash$table$[[$Investor$,$Marginal Combined Tax Rate,Taxable Rate of Return$],[$Sean$,40\%,5\%$