a math question

gmat2000

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.

answer D.

do not understand why (2) is sufficient... is the payment of $300 here a one time payment on one day or accumulative payments over a few days?... xiexie

lzhch89

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.

answer D.

do not understand why (2) is sufficient... is the payment of $300 here a one time payment on one day or accumulative payments over a few days?... xiexie

-- by 会员 gmat2000 (2011/3/24 8:54:56)

B的意思是前25天的平均数位540.
可以得出{600n+300(25-n)}/25=540, n=20. 说明在第20天存得这$300。

kathyyoung

楼上或楼主能翻译一下题目吗…………这题看不懂………………呜呜~~~~

kathyyoung

我是不明白（1）为什么可以…………能翻译一下吗，我看不懂题目……呜呜~谢谢！！！！

ozymendias

内牛满面 不懂，同求解释