Can you please explain and give formula in detail?

Duffy was just hired as CEO of Dog Food Inc. (DFI), which has no debt or its balance sheet. He also forecasts that the firm is expected to pay $200 million/year in taxes and is expected to have a constant 20% corporate tax rate forever. Assuming no bankruptcy costs, but personal taxes of 30% on income from equity and 50% on income from interest, if Duffy were to issue $100 million in perpetual debt at 8%, what is the present value of the tax shield? OA) - $12 million B) + $ 2 million OC) + $10 million OD) + $16 million E) + $30 million