Answer: She should pay in December
Explanation:
Assuming she pays in December, she can claim a 37% tax saving as it is an Expense and she will therefore pay the following Net of Tax,
= $20,000 * ( 1 - tax rate)
= 20,000 * ( 1 - 0.37)
= 20,000 * 0.63
= $12,600
Her total bill in December would be $12,600.
If she pays in January however then she would have lost 9% on the tax saving. Accounting for this would be,
The tax saving is
= $20,000 * 0.37
= $7,400
Discounting it to present day will be,
= 7,400 / (1 + r)
= 7,400 / 1 .09
= $6,788.99
Meaning that the amount she will effectively pay in January is,
= $20,000 - 6,788.99
= $13,211.01
Paying in December therefore saves her more.