Higgins Office Corp. Plans to maintain its optimal capital structure of 40 percent debt, 10 percent preferred stock, and 50 percent common equity indefinitely. The required return on each component source of capital is as follows: debt - 8 percent; preferred stock- 12 percent; common equity- 16 percent. Assuming a 40 percent marginal tax rate, what after tax rate of exchange must Higgins Office Corp. Earn on its investments if the value of the firm is to remain unchanged