The White Corporation has a capital structure of 60 percent common equity, 10 percent preferred stock, and 30 percent debt. This capital structure is believed to be optimal. To finance expansion plans over the coming year, the firm expects to have $40 million in retained earnings available. The cost of retained earnings is 18 percent. Additional common equity can be obtained by selling new common stock at a cost of 19.6 percent. Preferred stock can be sold at a cost of 15 percent. $25 million in secured bonds can be sold at a pretax cost of 14 percent. Beyond $25 million, the firm would have to sell unsecured bonds (debentures) at a pretax cost of 15 percent. The firm's marginal tax rate is 40 percent.

What are the firm's lowest and highest weighted costs of capital?

-- The answers are 14.82% for lowest and 15.96% for highest, but I'm having trouble finding this. --