有人能帮忙解释下这道PREP是到底在说什么吗？

carolcai

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.

答案是D，不大能理解题意。

jelen

这一个月的信用期，开始你有600，到了月末你还有300（也就是说这一个月之内你花了300）。the average daily balance是什么意思呢？举个例子，到第25天的average daily balance就是这25天以来每一天你账上的钱的平均数。题目要解的是这一个月30天每天账上的钱的平均数。

解这个问题的关键就是确定这个月唯一的一笔支出，300块钱是哪天花的。知道这个，就能知道平均每天的钱（the average daily balance）是多少。俩条件分别可以达成。

carolcai

多谢jelen啊，帮偶的大忙了，解了好几道