I know answer is debit balance.700, but I don't know why?

On 2 September J. Callaghan purchased on credit goods worth 1,000 from our business. A further purchase on credit of 600 was made on 12 September. On 17 September, J. Callaghan paid part of her debt in cash 1,200. She then purchased a further 400 of goods on 24 September, but returned goods worth 100 the following day. What is the balance on her account at the end of September? A credit balance of 700 A nil balance A debit balance of 700 None of the above