请问一道数学题

Rosemarry102

90.  8266-!-item-!-187;#058&005947
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.

a10285

题意是问一个月账户余额的平均值？
（1）可得出前20天$600,后10天$300, 可得答案~
（2）设 在前25天中的第n+1天付费的，则600*n+300*（25-n）=540*25
      算得n=5，即在6th天的payment，至此和（1）同，得出答案~

选D

Rosemarry102

谢谢~ o(∩_∩)o...