一道看不懂的DS题

On Jane's credit card account, the average daily balance for a 30-day billing cylce is average(arithmetic mean) of the daily balances at the end of the 30 days. At the begining 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 substracted from the account during the billing cycle, what was average daily balance on Jane's account for the billing cycle?

1) Jane's payment was credited on the 21st day of the billing cyle.

2) The average daily balance through the 25th day of the billing cycle was $540.

答案是D, each statement ALONE is sufficient.

请问这题怎么理解呢？

题的意思是：一个30天帐期的平均日余额是：30天每天结束时的余额的平均数。

所以在付款300前，每天余额都是600，之后每天余额都是300，知道哪天付款即可。

（1）显然可以知道

（2）设X天余额是600，Y天余额是300，有

X+Y=25

(600X+300Y)/25=540

明显看出是可求出唯一解的。

此处可解出X=20，其实跟（1）一样，也是第21天时付款的。

BTW：新GMATPREP的英语真够绕人的。

不看答案我也做不出

谢谢, 不过请问
2) The average daily balance through the 25th day of the billing cycle was $540.怎么理解

我觉得the 25th day 应该单单指第25这一天

如果像理解的在25天期间，应该是
The average daily balance through the 25 days吧

请指教

从语法上来讲似乎你说的对。

但是单讲第25天的平均日余额，从数学上如何理解呢？

我觉得这里只所以说the 25th day而不是the 25 days,是说这个第25天非别的25天，就是这个30天计帐期里的第25天。也就是说从第1天到这个第25天截至，平均日余额当然是25天的平均数。

我的英语不好，仅供参考。

open to discuss