跪求nn解答，OG11-DS141，OG11-DS153

sophia_zx

141.On Jane's credit card account, the average daily balance for an 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.       D

求教nn们此题什么意思？应怎样解答？

153.Does the integer k have a factor p such that 1<p<k

(1)k>4

(2)13!+2<=k<=13!+13             B 

请教，（2）的想法是怎样的？

falconcn

不是NN，不过可以解答一下：
141：the average daily balance其实就是30天里，每天天黑前，账户里面的数目的平均数。题干里面说道，该月只有一笔交易，就是还账单，但是没有说是那天还的。
条件（1）告诉了是第21天，所以前20天里，每天的都是600；后10天里，每天都是（600-300），所以可以算出；
条件（2）：可以设第x+1天还了账单，那么可以得到一个方程：600*x+300*(25-x)=540*25,可以解得x；进而得到该月的平均数；

153：
条件（1）：不可以；如果k是一个质数，比如说29，那么29只有1和29两个因子；
条件（2）：可以；因为（13！+2）必有一个因子是2，（13！+13）必有一个因子是13，中间数可以类推；因此，k必定存在；

sophia_zx

感谢赐教！