Richmond Clinic has obtained the following estimates for its costs of debt and equity at various capital structures:

Using this information, what should be the firm's optimal capital structure? (Hint: Calculate the corporate cost of capital at each structure. Note that the costs of debt given in this problem are after-tax costs. Also, note that data on component costs at alternative capital structures are not reliable in real-world situations.)

Percent debt	Cost of Debt	Cost of Equity
0%		16%
20%	6.6%	17%
40%	7.8%	19%
60%	10.2	22%
80%	14%	27%