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Calculating Cost of Debt Shanken NV issued a $30-$year, $10$ per cent semi$-$annual bond $7$ years ago. The bond currently sells for

$108$ per cent of its face value. The company's tax rate is $35$ per cent. Assume face value of the debt is $1,000.$

$($a$)$What is the pre$-$tax cost of debt?

$($b$)$What is the after$-$tax cost of debt?

$($c$)$Which is more relevant, the pre$-$tax or the after$-$tax cost of debt? Why?