The capital structure for McKesson is provided below. If the firm has a $3\mathrm{.}5\%$ after tax cost of debt, $6\%$ commercial loan rate, a $13\mathrm{.}5\%$ cost of preferred stock, and an $18\%$ cost of common stock, what is the firm's weighted average cost of capital $($WACC$)?[$a$]$

Note: format is xx$.$xx$\%$

Capital Structure $($in K$\text{'}$s$)$

Bonds $ $1,083$

Commercial Loans $ $2,845$

Preferred Stock $ $268$

Common Stock $ $3,681$