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标题: 虚心求教一道prep数学题 [打印本页]

作者: helenefu 时间: 2008-2-19 22:37
标题: 虚心求教一道prep数学题
Data sufficency:
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 begining of a certain 30-day billing cycle, Jane's credit 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

不懂这题的意思，也完全不知道这种题该怎么解呢？虚心求教中~~

作者: 透明微笑 时间: 2008-2-19 22:40

同上问。。。

作者: andiewang1 时间: 2008-2-19 23:32

idon't understand, ask NN's reply too.

作者: heslaw 时间: 2008-2-19 23:50

we need the average daily balance

according to the question, the average daily balance ....is the average of the daily balances at the end of each of the 30 days

so we need to know the balance on each of the 30 days

since only 1 transaction happened in the month (-$300)

For condition 1:

(1) Jane's payment was credited on the 21st day of the billing cycle

we know that the daily balance for day 21st then, which is 600-300=300, so for day 22-30, also 300

for day 1-20, 600. so the average= (300x9+600x21)/30= 510

(2) since only 1 transaction happened
and day1st- 25th average is $540, which is less than 600. which means that, the payment is some day before 25th

let the some day be x

so 600x+ 300*(25-x)=540x25

x=20, means, the payment was made on 20th day

the rest of the steps will be similar to condition 1

末了说一句，真TMD的操蛋题

不知道我这么理解对不对

作者: ciscogeek 时间: 2008-2-19 23:55

同意楼上，我看完题就选D了。看来各人对题材的敏感度不一样。

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