Answer:
Pretax    =  5.61%
After tax = 4.26%
Explanation:
The cost of debt will be the Yield to maturity of the bonds. 
91 = present values of the 25 year annuity + present value of the maturity 
There is no formula for exact YTM
we can either use excel or calculate by approximation:
In this case we will calcualte the YTM by aprroximation
 
 
C=	25 cuopon payment 1,000 x 5% / 2 becayse paymenr are semiannually
F=	1000 the face value is 1,000
P=	910  the present value or market value is 91% of the face value
n=	50   25 year at 2 payment per year
 
 
dividend	26.8
divisor	955
YTM	5.6125654%
This will be the pretax cost of debt
then we calculate the after tax cost of debt
pre-tax cost of debt ( 1 - t ) = after-tax
5.61% ( 1 - .24 ) = 4,2636