Isabel, a calendar-year taxpayer, uses the cash method of accounting for her sole proprietorship. In late December she received a $20,000 bill from her accountant for consulting services related to her small business. Isabel can pay the $20,000 bill anytime before January 30 of next year without penalty. Assume her marginal tax rate is 40 percent this year and next year, and that she can earn an after-tax rate of return of 8 percent on her investments.