ST$4.$ Walther Enterprises has a capital structure target of $60$ percent common equity, $15$ percent

preferred stock, and $25$ percent long$-$term debt. Walther's financial analysts have estimated the

marginal, after$-$tax cost of debt, preferred stock, and common equity to be $9$ percent, $15$ percent, and $18$

percent, respectively. What is the weighted marginal cost of capital for Walther?