Here is another alternative solution to the question (I know the attached file might be crude but I hope it helps)
Recall that the only item that can increase debtors balance is credit sales and the only item that can also reduce it is cash/cheque from customers .
Therefore ,debtors ledger account can be opened in order to ascertain the credit sales .As shown in the file,debit the account with opening balance of debtors in 2004 ;3,140 while you creditor with cash deposit from customers ;15,270 and the closing balance of debtors in 2004 (which becomes the opening balance in 2005).Then your balancing figure on the debit side which was asterisked in the file ( derived by deducting the opening balance from the summation of cash and closing balance i.e 19,350(15,270 + 4,080) - 3,140 = 16,210 ) is your credit sales .
Though this is a long shot but it's just for clarity.