effective interest rate

A discount electronics store advertises the following financing arrangement: “We don’t offer you confusing interest rates. We’ll just divide your total cost by 10 and you can pay us that amount each month for a year.” The first payment is due on the date of sale and the remaining eleven pay- ments at monthly intervals thereafter. Calculate the effective annual interest rate the store’s customers are paying on their loans.

P.S.The ans is 51.2%. I don't know how to do.