数学一道 求讲解 急急急

guo573874201

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.答案选D  为什么条件2可以呢  条件2是说在25号的时候300元已经花了  那么 具体那一天不知道啊 反正是在25号之前  怎么能求出呢？

supporthugo

首先要正确的了解题意。我们所需要求的是一个30天内帐户余额的平均数
接着看条件
条件一原题告诉我们那笔300元的payment是在21号付掉的，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。
条件二告诉我们前25天的平均数是540，说明300元的payment肯定是在钱25天付掉的，否则平均数肯定是600元，所以说最后后5天卡内的金额为300元，然后还是算下平均 （540×25＋300×5)/30 。
因此可以得出两个条件都可以单独推出答案，所以选D
希望我的解释可以帮到兄台，祝你考出理想的成绩。加油！Hold on！

xinnono

条件2是说 (600*n+(600-300)*(25-n))/25=540, 解得n=20, 所以第21天刷的卡~ 1, 2 各自都可以得到正解, 所以D