Kim Davis is in the 40 percent tax bracket.

Kim Davis is in the 40 percent tax bracket. She is considering investing in HCA (taxable) bonds that carry a 12 percent interest rate. 
a. What is her after-tax yield (interest rate) on the bonds? 
b. Suppose Twin Cities Memorial Hospital has issued tax-exempt bonds that have an interest rate of 6 perecent. With all else the same, should Kim buy the HCA bonds (earlier question) or the Twin Cities bonds?