请教：Prep-DS-1-90

Braveboy2008

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.

为什么 （2） 也对？ 只告诉你了25号之前的情况， 没有说300 在后5天中的那一天付款的呀，怎么能够确定 daily balance???

坚定之路

呵呵，LZ请看：前25天的平均余额是540元，低于月初的600元，说明那个300元已经在前25天中的某一天被还掉了；还款的动作只执行一次的。

Braveboy2008

thank you, GMAT math is not easy !!!!