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

Percent Debt: 0% , 19.3 , 40.3 , 60.5 , 79.4

After-Tax Cost of Debt: -- , 6.8% , 7.7 , 10.3 , 14

Cost of Equity: 14.4% , 15.2 , 16.6 , 19.3 , 27.9

What is the firm's cost of capital when it is at its optimal capital structure? (Hint: Calculate its corporate cost of capital at each structure.)