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

Percent Debt	After-Tax Cost of Debt	Cost of Equity
0%	-----	16%
20%	6.6%	17%
40%	7.8%	19%
60%	10.2%	22%
80%	14.0%	27%

What is the firms optimal capital structure? (Calculate its corporate cost of capital at each structure)