求救Prep里一道Sufficiency题

HYoShin

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

答案是说选D。
可能是我连题目都没能看懂的原因吧，我总觉得那两个都不充分去说明那个Average Daily Balance啊。希望大家指点指点吧。这个题目让我纠结了好久呢。