The Richardson Oil Company is considering issuing additional debt. They wish to use the yield on their existing debt as a guide to the cost of new debt. They currently have a zero-coupon bond outstanding that has five years to maturity and a current market price of $74 \%$, or $$\$ 747.50$$ per $$\$ 1,000$$ par value.
a. If Richardson's marginal tax rate is $20 \%$, what is the cost of debt?
b. If Richardson's marginal tax rate is $30 \%$, what is the cost of debt?