[問題]PREP-T1-Q90 (DS)

[scottkidd]

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:D

不太看得懂題目，可否幫忙講解一下，請問(1)為什麼充份，謝謝！

[eced]

前面都在解譯 帳戶是取三十天 每天的餘額 平均來計算, 起先有帳戶有六百元 中途付掉了300元,求平均是多少
(1)第21天付掉300, 前20天的餘額都是600, 21天後的餘額都剩300,所以平均可得
(2)前25天的餘額是540, 表示300在其中某天付掉了,設一下方程式可求得在那天付了300元,然後可得平均

[dibert8]

(1) (600*20 + 300*10) / 30
(2) (540*25 + 300*5) / 30

[Austing]

(1) (600*20 + 300*10) / 30
(2) (540*25 + 300*5) / 30[/quote]
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Will u kindly explain for me that 300 is definitely paid after the 540(25th day)??

really appreciate;)

[Austing]

finally figure it out~~~ccc
因為除了300payment外沒有任何支出或收入
1st-----------600
25th---------540
有減少所以一定是表示在這幾天付掉的
那他付完以後就是300了
所以後面幾天就是300

[chris8888]

我認為是這樣 :

(1). 21th credited 表示付掉300元, 意思就是說
　　 (600 * 20 + 300 * 10)/30 = the average daily balance on Jane's account for the billing cycle.

(2). The average daily balance through the 25th day of the billing cycle was $540.
　　假設Xth credited.
　　(600 * X + 300(25-X))/25 = 540
　　可以算出X=20
　　因此 (600 * 20 + 300 * 10)/30 = the average daily balance on Jane's account for the billing cycle.