扫描二维码，直接登录
题库 / Prep2007E1-DS-90
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.
Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
EACH statement ALONE is sufficient.
Statements (1) and (2) TOGETHER are NOT sufficient.
The average daily balance through the 25th day of the billing cycle was $540. 第25天的日均余额是540.说明300块已经被划走了，(25*540+300*5)/30
（2）前25天的平均值为540，说明付款肯定已经发生，后5天的约肯定为300.总的数额为540*25+300*5
告诉你在最初时候帐户里有600块，然后这三十天中帐户唯一的变化就是某天用掉了300。
条件1就他说那个钱是第21天付的，那就直接（20×600＋10×300)/30。就是说前20天帐户里都是600块，后十天都是300块，算个平均就行了。
条件2说前25天的平均数是540。那说明那300块肯定是在前25天中的某天中付的，否则平均数应该是600。设 有x天用300；x 300+(25-x)600=25乘540 x=5 最后求出( 5*300+25*600 )/30
两个条件都能算。选d, either。
信用卡放那平时都不花的吗
做数学题也是不能加自己主观背景
到底是什么
1. 第21天花的300.那前20天都是600，后10天300 可以算
2 前25天平均余额是540，说明300在前25天花的。方程求出哪天花的 能算
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.
the average daily balance for a 30-day billing cycle = [30*daily balances at the end of each day]/30
期初余额是600，在三十天内有一笔300的支出
要求平均余额数，就是需要知道 这一笔 300的支出，是在哪一天发生的，因为这一笔支出发生在哪一天，从那天起每天的账户余额变成300
（1）已知某天付出300，那么在这之前是600，之后是300，平均值很好算出来。
（2）已知某天的平均值，因为小于600，可知在这期间已经付出300，之后的余额均为300，结合所给的平均值，也可以算出总的平均值。
同求登录 或 注册 后可以参加讨论