Thunderhorse Oil is a U.S. oil company. Its current cost of debt is 7.30​%, and the​ 10-year U.S. Treasury​ yield, the proxy for the​ risk-free rate of​ interest, is 3.00​%. The expected return on the market portfolio is 8.20​%. The​ company's effective tax rate is 40​%. Its optimal capital structure is 75​% debt and 25​% equity.
a. If​ Thunderhorse's beta is estimated at 1.30​, what is​ Thunderhorse's weighted average cost of​ capital?
b. If​ Thunderhorse's beta is estimated at 0.90​, significantly lower because of the continuing profit prospects in the global energy​ sector, what is​ Thunderhorse's weighted average cost of​ capital?