FNB *****ELENT INTEREST RATE CHARGED 21% BUT MORE LIKE 70%

I've had so much of issues with FNB and would like to make others aware one must check the interest rate that FNB is charging clients. I had a balance outstanding of R321.60 and they stipulated their interest rate is 21% but if you multiply R321.60 x 21% the total is R67.54 even if they are charging you double it's supposed to have been R135.08 meaning they are charging for 55days. No one on fnb can help or give you answers as they don't know how to calculate it themselves and give you a million excuses.