[求助]急问一道prep数学，谢谢

On Jane's credit card account, the average daily balance for a 30-day billing cylce is average(arithmetic mean) of the daily balances at the end of the 30 days. At the begining of a certain 30-day billing cycle, Jane's credit card account had a balance of $600. Jane made a payment of $300 on the account during the billing cycle. If no other amounts were added to or substracted from the account during the billing cycle, what was average daily balance on Jane's account for the billing cycle?

1) Jane's payment was credited on the 21st day of the billing cyle.

2) The average daily balance through the 25th day of the billing cycle was $540.

答案是D, each statement ALONE is sufficient.

请问这题怎么理解呢？