A large retailer obtains merchandise under the credit terms of 3/20, net 45, but routinely takes 60 days to pay its bills. (Because the retailer is an important customer, suppliers allow the firm to stretch its credit terms.) What is the retailer's effective cost of trade credit? Assume 365 days in year for your calculations. Do not round intermediate calculations. Round your answer to two decimal places.

Please explain the math, I don't understand how the other examples get to 10 days, etc. Is this just standard?