Seduak has estimated the costs of debt and equity capital for various proportions of debt in its capital structure.

% Debt After-tax cost of debt Cost of equity

0% - 13.0%

10 5.4% 13.3

20 5.4 13.8

30 5.8 14.4

40 6.3 15.2

50 7.0 16.0

60 8.2 17.0

Based on these estimates, determine Seduaks optimal capital structure