请教prep 中数学题3-1

小货舱

DS: On Jane's credit card account, the average daily balance for a 30-day billing cycle is the average(arithemetic mean) of the daily balances at the end of the 30-days. At the begging 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 substract 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 was $540

实话道 我这道题题目都看不明白，请各位同学不吝赐教！

小货舱

答案是 D

小货舱

跪求各为NN献点宝贵的"鸡血"吧