[讨论]PREP数学题目的综合讨论帖

sinowen

以下是引用冰岛哲学C在2007-12-15 21:58:00的发言：

当Y=13...X=3的时候。.也满足啊~`

不过哪位NN能不能解释下.考场上是不可能验算完这么多的数的吖 ~~

有没啥办法排除: 因为要求整数解而可能使二元一次方程也能得出唯一满足题干假设的解呢?

当Y=13时，X=3时，15x+25y=370,不等于420呀......

冰岛哲学C

以下是引用sinowen在2007-12-16 10:41:00的发言：

当Y=13时，X=3时，15x+25y=370,不等于420呀......

不好意思吖~~写错了.

是Y=15..X=3

sinowen

以下是引用冰岛哲学C在2007-12-16 11:28:00的发言：

不好意思吖~~写错了.

是Y=15..X=3

有道理，顶一个，忘了这么一个大东西！

sinowen

102. 

If M is a finite set of negative integers, is the total number of integers in M an odd number?

(1)  The product of all the integers in M is odd.

(2)  The product of all the integers in M is negative.

答案是B，我选择E。

107. 

A combined total of 55 lightbulbs are stored in two boxes; of these, a total of 7 are broken.  If there are exactly 2 broken bulbs in the first box, what is the number of bulbs in the second box that are not broken?

(1)  In the first box, the number of bulbs that are not broken is 15 times the number of broken bulbs.

(2)  The total number of bulbs in the first box is 9 more than the total number of bulbs in the second box.

答案是D，我选择A。

113. 

For a certain set of n numbers, where n > 1, is the average (arithmetic mean) equal to the median?

(1) If the n numbers in the set are listed in increasing order, then the difference between any pair of successive numbers in the set is 2.

(2) The range of the n numbers in the set is 2(n - 1).

答案是A，我选择C.

119. 

A contractor combined x tons of a gravel mixture that contained 10 percent gravel G, by weight, with y tons of a mixture that contained 2 percent gravel G, by weight, to produce z tons of a mixture that was 5 percent gravel G, by weight.  What is the value of x ?

(1) y = 10

(2) z = 16

答案是D，我选择A。

128. 
The points A, B, C, and D are on a number line, not necessarily in that order.  If the distance between A and B is 18 and the distance between C and D is 8, what is the distance between B and D ?

(1) The distance between C and A is the same as the distance between C and B.

(2) A is to the left of D on the number line.

答案是E，我选择C。

131. 
The positive integer k has exactly two positive prime factors, 3 and 7.  If k has a total of 6 positive factors, including 1 and k, what is the value of k ?

(1) 32 is a factor of k.

(2) 72 is not a factor of k.

答案是D，我选择C。

147. 
Each employee on a certain task force is either a manager or a director.  What percent of the employees on the task force are directors?

(1) The average (arithmetic mean) salary of the managers on the task force is $5,000 less than the average salary of all employees on the task force.

(2) The average (arithmetic mean) salary of the directors on the task force is $15,000 greater than the average salary of all employees on the task force.

答案是C，我选择E。

148. 
In the xy-plane, does the line with equation y = 3x + 2 contain the point (r, s) ?

(1)  (3r + 2 - s)(4r + 9 - s) = 0

(2)  (4r - 6 - s)(3r + 2 - s) = 0

答案是C，我选择D。

sinowen

得，先自己顶一下，今天有些不在状态，只整到175题。接下来还有一套CD要看。。。加油！！！

sinowen

158. 

Each of the 25 balls in a certain box is either red, blue, or white and has a number from 1 to 10 painted on it.  If one ball is to be selected at random from the box, what is the probability that the ball selected will either be white or have an even number painted on it?

(1)  The probability that the ball will both be white and have an even number painted on it is 0.

(2)  The probability that the ball will be white minus the probability that the ball will have an even number painted on it is 0.2.

答案是E，蒙对的，但是真地不清楚做题的思路。

peter3008

158.
Each of the 25 balls in a certain box is either red, blue, or white and has a number from 1 to 10 painted on it.  If one ball is to be selected at random from the box, what is the probability that the ball selected will either be white or have an even number painted on it?
(1)  The probability that the ball will both be white and have an even number painted on it is 0.
(2)  The probability that the ball will be white minus the probability that the ball will have an even number painted on it is 0.2.
答案是E，蒙对的，但是真地不清楚做题的思路。

-- by 会员 sinowen (2007/12/19 21:33:00)

P(AUB)=P(A)+P(B)-P(A&B)
A=white, B=even

(1):告诉我们 P(A&B)=0,但是P(A),P(B)并不知道，因为，25个球中有红，蓝，白多少个，1-10有多少个没有办法球，如果告诉了（我假设 25个中白球有x个，even 有y个），式子可以那么写：
P(A)=C(1,x)/C(1,25)  ,P(B)=C(1,y)/C(1,25)

(2): P(A)-P(B)=0.2， 把P(A)和P(B)看作是两个未知数，P(AUB)=P(A)+P(B)-P(A&B) 中还有其他2个未知数不知道，insuff

（1）+(2)  同样不行，理由与(1)一样

（E）

weizi8855

The residents of Town X participated in a survey to determine the number of hours per week each resident spent watching telecision . The distribution of the results of the survey had a mwan of 21hours and a  standard deviation of 6 hours. The number of hours that Pat, a resident of Town X watched television last week was between 1 and 2 standard deviations below the mwan ,which of the following could be the number of hours that Pat watched television last week.
选项 :30   20  18  12   6


对于Standard deviation 问题不是很清楚,帮忙解答7一下吧~~~~~~

hermy

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？

-- by 会员 sinowen (2007/12/15 11:17:00)

这题我也困惑啊，总不能因为只用2就得出1/8这个数字就说2是对的吧？？

清幽的雨

20.   1491-!-item-!-187;#058&000755
可以只用条件2解啊，除去7/8的人是下午的session之外，剩下的就只是上午session的人了（用韦恩图可以看得更清楚），所以1/8*128=16人只参加了上午的session