问几道PREP数学（周四考试）

hersaysis

84.7517-!-item-!-187;#058&005572

If p is a prime number greater than 2, what is the value of p ?



(1)There are a total of 100 prime numbers between 1 and p + 1.



(2)There are a total of p prime numbers between 1 and 3,912.

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【答案】D

【思路】

(1) 從1開始署到第101(2不是)的質數即是p←選項(1)成立

(2) 數1~3912有幾個質數，即可得知p ←選項(2)成立

我的问题： （2）怎么能保证 1到3912之间质数的个数肯定为质数呢？总不能一个一个数吧？

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.

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【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

问题： 选项（2）看不懂，如果(D1+...D25)/25=540, 如果300是在这25天内支付的，不是应该为(D1+...D25)/25=（600-300）吗？

谢谢~~~~
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.

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【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

问题： 选项（2）看不懂，如果(D1+...D25)/25=540, 如果300是在这25天内支付的，不是应该为(D1+...D25)/25=（600-300）吗？

谢谢~~~~

Jaycandoit

84. 题目是已知p是质数了，只是问你能不能求p的值，所以你只要确定（2）是可以求出来p的值就行，不用理会p是不是质数，也不需要你求p的值

90. balance through the 25th day和payment was credited on the 25th day是两个概念，前者表示前25天的平均数，后者表示在25号payment减300， 前25天是平均540，所以前25天的总balance是25*540，因为540比600小，所以payment credited date肯定是在25号之前的，因此剩下的5天balance都只有600-300=300

mayubo_1129

(2)你的理解有问题，题目是说1-3912之间一共有P个质数，1-3912之间质数的数量是一定的，所以，P也是一定的。。。

hersaysis

谢谢啦~