WACC Kose SA has a target debt$$equity ratio of $0\mathrm{.}80.$ Its WACC is $10\mathrm{.}5$ per cent, and the tax rate is $35$ per cent.

$($a$)$ If Kose$$s cost of equity is $15$ per cent, what is its pre$-$tax cost of debt?

$($b$)$ If instead you know that the after$-$tax cost of debt is $6\mathrm{.}4$ per cent, what is the cost o$?$