Prep-DS-90

youfee

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.
请问大家这道题到底什么意思啊，当时做了许久几乎崩溃了，好像Og上也有过类似的，当时也没做对

youfee

没人理我？55555555

s

回复 2# youfee


    意思就是每天的账户余额在30天的平均值，即每天的账户余额的和除以30
（1）第21天支付300，前20天都是600,20~30天是600-300=300
       平均值就是（600*20+300*10）/30
(2) 设在x+1天支付300  
    (600x+300(25-X)）/25=540  X=20  同（1）
所以这个T选D

caicainiao

回复 1# youfee


    找个官方答案

【答案】D
【思路】平均值= (每天的餘額平均，昨天的期末於今天的期初餘額)，求平均值。已知:1.d1=600，其中dn=-300。且其中無任何金額變動。

(1) 300元是在第21天中付出，所以可得知1~20天每日餘額是600，21~30每日餘額是300即可求總平均值 ←選項(1)成立
(2) =540  且300是在第25天以前付掉(因期初為600)，第26~30天每日餘額為540，即可求總平均值， ←選項(2)成立