Solution :
It is given that Christopher is the cash method and a calendar year taxpayer. He also made the cash payments that is related to the business for this year.
We have to assume marginal tax rate = 30 %
Therefore, the after tax cost for the payments are :
a). $ 500 - not deductible as it was a penalty for the violation.
b). 765 - half of the interest is not deductible. i.e. 900 x (1 - (0.5 x 0.3))
c). 420 - fully deductible, i.e. 600 x (1 - 0.3)
d). 450 - not deductible