已解。求解释一道数学题，题目没看懂

Seajinking

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.

这道题题目没看懂，求高人解释一下，给个答案。