[讨论]PREP数学题目的综合讨论帖

首先非常感谢elmerden的强大support, 现在数学在有JJ的同时，还能有完整的PREP的题目和答案进行分析。

但是相信许多数学不是很NN的战友，包括我在内，还是有一些题目不是很清楚，于是开了这个贴子，

把自己认为有些疑惑的问题拿来和大家分享。求抛来的砖，求引来的玉。

如果刚刚进入复习阶段的朋友，可以通过这个贴子的一些有代表的题目省去许多不必要的重复，愿望如斯 。。。。。。

（更新到150题了）

3.    196-!-item-!-187;#058&000141

How many different prime numbers are factors of the positive integer n ?

 (1) Four different prime numbers are factors of 2n.

 (2) Four different prime numbers are factors of n².

答案是B，但是我却没有什么思路去理解。

4.    250-!-item-!-187;#058&000160

Is the product of a certain pair of integers even?

(1)  The sum of the integers is odd.

(2)  One of the integers is even and the other is odd.

 答案是D，我选择的是B。比如几个奇数的和是奇数，而且它的乘积也是奇数呀；3,5,7,和为15，积为105.

10.   719-!-item-!-187;#058&000515

Of the 800 students at a certain college, 250 students live on campus and are more than 20 years old.  How many of the 

 800 students live on campus and are 20 years old or less?
      

 (1)  640 students at the college are more than 20 years old. 

 (2)  60 students at the college are 20 years old or less and live off campus.  

答案是C，这种题目一直是我的软肋，真是没有思路。。。通常我都是选E，但是好像答案基本都是前面四个。 

11.   959-!-item-!-187;#058&000583

If p is a positive odd integer, what is the remainder when p is divided by 4 ?

(1) When p is divided by 8, the remainder is 5.

(2) p is the sum of the squares of two positive integers.

答案是D，我选择的是A。因为任意两个数的平方的和如果是奇数的话，除以4应该是变化在1和3之间的。

17.   1329-!-item-!-187;#058&000662

Six countries in a certain region sent a total of 75 representatives to an international congress, and no two countries sent the same number of representatives.  Of the six countries, if Country A sent the second greatest number of representatives, did Country A send at least 10 representatives?

(1)  One of the six countries sent 41 representatives to the congress.

(2)  Country A sent fewer than 12 representatives to the congress.

答案是E，我选择是A。因为75-41，就剩下34个代表了。等于就是剩下的五组分；而A组又是第二多的。假设A的代表为9，则剩下25个代表由四个代表队分，而且不能重复，则造成无解（或是有解，我没有想到。）

19.   1437-!-item-!-187;#058&000677

Is z equal to the median of the three positive integers x, y, and z ?

(1) x < y + z

(2) y = z

答案是B，我选择是E。我把X,Y,Z分别看成是三角形的三个边来解的。

PREP2.-44

The arithmetic mean and standard deviation of a certain normal distribution are 13.5 and 1.5, respectively.  What value is exactly 2 standard deviations less than the mean?

(A) 10.5

(B)  11.0

(C)  11.5

(D) 12.0

(E) 12.5

完全没看懂题目+没思路....

求教吖~~

20.   1491-!-item-!-187;#058&000755

A certain one-day seminar consisted of a morning session and an afternoon session.  If each of the 128 people attending the seminar attended at least one of the two sessions, how many of the people attended the morning session only?

(1)3/4 of the people attended both sessions.

(2) 7/8of the people attended the afternoon session.

答案是B，我选择是C。因为7/8里包含了两个课程都上的呀，怎么可以单独求出解呢？