I have an account wi...

I have an account with truworths a 6month pay back plan that is interest free my balance after paying my account is R1500 I still owe now I got my statement too pay for September is R630 now if you take R630 x 6months is R3 780 how in the hell if it's an interest free account and I only owe R1500 I must pay R630 a month this people have no clue what they are doing the accounts department don't know who too take R1500 and ÷ it by 6 if you can't count you should not be in accounts department