标题: 急切，请教一题DS，大侠帮忙，我题目都没看懂！！ [打印本页] 作者: andreasyang 时间: 2010-8-26 16:14 标题: 急切，请教一题DS，大侠帮忙，我题目都没看懂！！ 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.作者: HelenW 时间: 2010-8-26 16:19
选d？ 两个都可以作者: nicktian 时间: 2010-8-26 16:23
楼上正解作者: qsy66 时间: 2010-8-26 16:27
不懂，请解释作者: andreasyang 时间: 2010-8-26 16:32
是D ，为啥？？作者: nicktian 时间: 2010-8-26 16:38
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，也就是题目所问了作者: HelenW 时间: 2010-8-26 16:49
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