[求助]GMATprep中的一个math

On Jane's credit card account,the average daily balance for 30-day billing cycle is the    average(arithmetic mean)of the daily balance 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 during the billing cycle.If no other accounts            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.

What I want to know is the meaning of the first sentence.

Thanks a lot.

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