一道PREP数学请求帮助

audrey1007

On Jane's credit card account, the average daily balance for a 30-day billing cycle is the average (arithmetic mean) of the

daily balances at the end of each of the 30 days.  At the beginning 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 subtracted from the account during the billing cycle, what was the average daily balance on

Jane's account for the billing cycle?

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

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

此题选D，但我不明白第2个条件怎么推