Answer:
A) Isabel's after-tax cost for paying the bill in December = $19,000 - ($19,000 x 40%) = $19,000 - $7,600 = $11,400
B) Isabel's after-tax cost for paying the bill in January:
the cost before taxes = $19,000 - ($19,000 x 4%/12) = $19,000 - $63 = $18,937
after-tax cost = $18,937 - ($18,937 x 40%) = $18,937 - $7,575 = $11,362
C) January, since the cost of the debt is lower.
