UB is examining its capital structure with the intent of arriving at an optimal debt ratio. It currently has no debt and a beta of $1\mathrm{.}5.$ The riskless interest rate is $9\%$ and market premium is $5\mathrm{.}5\%.$ Your research indicates that the debt ratings will be as follows at different debt levels:

Debt$/($Debt $+$ Equity$)\%$ Ratings Interest Rate $(\%)$

$0$ AAA $10$

$10$ AA $10\mathrm{.}5$

$20$ A $11$

$30$ BBB $12$

$40$ BB $13$

$50$ B $14$

$60$ CCC $16$

$70$ CC $18$

$80$ C $20$

$90$ D $25$

The firm currently has $1$ million shares outstanding at $$20$ per share $($tax rate is $40\%)$

What is the firm optimal debt ratio?