I got the formula from lecture slides but when I tried to put those numbers into the calculator, why it showed a different answer?

a. P14 Your firm purchases goods from its supplier on terms of 3/15, net 40. What is the effective annual cost to your firm if it chooses not to take the discount and makes its payment on day 40? Effective Annual Cost of not taking cash discount (1+Discount rate/(100%- Discount))^365/(Number of days of credit-discount period)-1 =(1+3%/(100%- 3%))^(365/(40-15))-1 =56.00% b. What is the effective annual cost to your firm if it chooses not to take the discount and makes its payment on day 50