The component cost would be the after tax YTM on the existing debt. The YTM is the discount rate that would make the present value of interest and principal equal to the price today.
The interest is paid quarterly = 80/4 = $20, period = 25X4=100 quarters
900.90 = 20 X PVIFA (100, rate) + 1,000 X PVIF (100,rate)
YTM = 9%
The component cost of debt = 9% X (1-0.4) = 5.4%