Marginal Incorporated $($MI$)$ has determined that its after$-$tax cost of debt is $9\mathrm{.}0\%.$ Its cost of preferred stock is $13\mathrm{.}0\%.$ Its cost of internal equity is $18\mathrm{.}0\%,$ and its cost of external equity is $20\mathrm{.}0\%.$ Currently, the firm's capital structure has $$324$ million of debt, $$60$ million of preferred stock, and $$216$ million of common equity. The firm's marginal tax rate is $35\%.$ The firm is currently making projections for the next period. Its managers have determined that the firm should have $$82$ million available from retained earnings for investment purposes next period. What is the firm's marginal cost of capital at a total investment level of $$278$ million?