A companys current capital structure is 50% debt and 50% equity. The cost of debt is 6.3% per year. The companys beta is 1.2 and the tax rate is 32%. If the debt ratio is increased to 65%, the cost of debt will increase to 7.0% per year. If the debt ratio is decreased to 40%, the cost of debt will decrease to 6.0% per year. The risk-free interest rate is 3.1% per year and the market risk premium is 5.5% per year. What is the companys optimal capital structure?