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

Base on these estimates, determine Seduak's optimal capital structure.