Annual Compound Interest Rate Question

Just did CIMA's pearson vue F1 mock, and there was some stuff that came up that I don't think's been covered in my text books. Please could someone help with the following question (I'm confused by it); An entity has been offering 60 day payment terms to customers, but wants to reduce its working capital and improve cash flow. The entity is proposing to offer 1.5% discount for payment within 28 days. Assume an invoice of $1000. Calculate the effective annual compound interest rate that will be incurred if this proposal is adopted. Give your answer to one decimal place (%).