There is no 20% tax bracket for federal taxes.
So, I must conclude this is a homework question.
You did get the 6 month payment right. Remember to subtract out the interest paid to the seller.
As for the discount, how are you planning to handle that? With taxable bonds, did you have a choice? Give us the answer you found when you read IRS publication 550.
Edit, I see TRO either didn't read the question or didn't understand it. Par for the course.