Takeoka's Sushi To Go has estimated the following cost of debt (before-tax) and cost of equity.

Proportion of DebtBefore-tax Cost of DebtCost of Equity

30%9.5%14.4%

40%9.7%15.2%

50%10.1%16.2%

60%11.8%18.5%

What is the cost of capital at Takeoka's optimal capital structure given the above information and a 40% effective tax rate?