急切，请教一题DS，大侠帮忙，我题目都没看懂！！

andreasyang · 发表于 2010-8-26 16:14:51

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.

HelenW · 发表于 2010-8-26 16:49:24

题目：平均每天的余额是30天每天余额的平均值，30天中初期的余额是600 但期间花了300 中间没有别的收入和支出问平均余额
1）=（20*600+10*300）/30
2) = (540*25+300*5)/30 因为前面均值为540 所以在前25天中已经支了300 剩下的5天每天余额也就是300了

nicktian · 发表于 2010-8-26 16:38:04

就是有两个概念，一个是30天周期的账户余额平均数，一个是每天的账户余额。刚开始有600，用了300后的当天就变成了300的余额（600-300），两个阶段，前面的天数一直是600，后面变成300，未知的是什么时候支出的这笔钱，也就是题目所问了

andreasyang · 发表于 2010-8-26 16:32:15

是D ，为啥？？

qsy66 · 发表于 2010-8-26 16:27:35

不懂，请解释

nicktian · 发表于 2010-8-26 16:23:26

楼上正解

HelenW · 发表于 2010-8-26 16:19:13

选d？两个都可以

急切，请教一题DS，大侠帮忙，我题目都没看懂！！

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