The treasurer of a firm noted that many invoices were received with the following terms of payment:"2 percent- 10 days, net 30 days". Thus if she were to pay the bill within 10 days of its data, she could deduct 2 percent . on the other hand , if she did not promptly pay the bill, the full amount would be due 30 days from the date of the invoice. Assuming a 20 day compounding period, the 2 percent deduction for prompt payment is equivalent to what effective annual interest rate?