I seem to be missing something so steps would be great so I can understand what I am possibly missing. Thank you.

Isabel, a calendar-year taxpayer, uses the cash method of accounting for her sole proprietorship. In late December she received a $30,000 bill from her accountant for consulting services related to her small business. Isabel can pay the $30,000 bill anytime before January 30 of next year without penalty. Assume her marginal tax rate is 37 percent this year and next year, and that she can earn an after-tax rate of return of 4 percent on her investments. a. What is the after-tax cost if Isabel pays the $30,000 bill in December? After-tax cost $ 18,900 b. What is the after-tax cost if Isabel pays the $30,000 bill in January? Use Exhibit 3.1. (Round your answer to the nearest whole dollar amount.) After-tax cost EXHIBIT 3-1 Present Value of a Single Payment at Various Annual Rates of Return 4% 5% 6% 7% 8% 9% 10% 11% 12% Year 1 962 952 943 935 926 917 909 901 .893 Year 2 .925 907 .890 .873 .857 842 .826 .812 .797 Year 3 .889 .864 .840 .816 .794 .772 .751 .731 .712 Year 4 .855 .823 .792 .763 .735 708 .683 .659 .636 Year 5 .822 .784 -747 .713 .681 .650 .621 .593 567 Year 6 .790 .746 .705 .666 .630 .596 .564 535 -507 Year 7 .760 .711 .665 .623 583 547 .513 .482 452 Year 8 .731 .677 .627 .582 540 502 467 .434 404 Year 9 .703 .645 .592 544 500 460 424 391 361 Year 10 .676 .614 .558 .508 463 422 386 352 322 Year 11 .650 .585 527 475 429 388 350 317 .287 Year 12 .625 557 497 444 397 356 319 .286 .257 Year 13 .601 .530 .469 415 368 326 .290 .258 .229 Year 14 577 .505 442 388 340 .299 .263 .232 .205 Year 15 555 .481 417 362 315 .275 .239 -209 .183