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Calculate Interest rate on debt

I’m looking at this page, which explains the valuation of equity as a call option. I can calculate the value of the call and the value of the outstanding debt using the black and Scholes formula. I don’t understand the formula to calculate the Interest rate on debt.

It says:

($ 80 / $24.06)1/10 -1 = 12.77%

but I do not see how this formula gets to 12.77%

This is in the source:

The parameters of equity as a call option are as follows:

Value of the underlying asset = S = Value of the firm = $ 100 million

Exercise price = K = Face Value of outstanding debt = $ 80 million

Life of the option = t = Life of zero-coupon debt = 10 years

Variance in the value of the underlying asset = s2 = Variance in firm value = 0.16

Riskless rate = r = Treasury bond rate corresponding to option life = 10% Valuing Equity as a Call Option

Based upon these inputs, the Black-Scholes model provides the following value for the call:

d1 = 1.5994 N(d1) = 0.9451

d2 = 0.3345 N(d2) = 0.6310 Value of the call = 100 (0.9451) - 80 exp(-0.10)(10) (0.6310) = $75.94 million

Value of the outstanding debt = $100 - $75.94 = $24.06 million

Interest rate on debt = ($ 80 / $24.06)1/10 -1 = 12.77%

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