For this question, I want to know the formula and definition to calculate the annual rate. And why the answer said it should be divided by 30 days?(I think the payment should within 15 days to get the discount)

A company purchased goods on credit with credit terms of 3/15, n/45. Although the company does not have cash available to pay within the discount period, the manager of the company is considering borrowing money to take advantage of the discount. In order to make the appropriate decision, the manager computed the annual interest rate associated with the sales discount. What is the approximate annual rate? Multiple Choice 25%. O 18%. 56% 38%. Explanation Calculation: [($3-$97) = 30 days) * 365 days = 38