在cd建一个错题集，欢迎交流

hahouhei · 发表于 2016-9-15 19:56:33

For every positive even integer n, the function h(n) is defined to be the product of all the even integers from 2 to n, inclusive. If p is the smallest prime factor of h(100) +1, then p is?
A. between 2 and 20
B. between 10 and 20
C. between 20 and 30
D. between 30 and 40
E. greater than 40

h(100)+1=2∗4∗6∗...∗100+1=250∗(1∗2∗3∗..∗50)+1=250∗50!+1h(100)+1=2∗4∗6∗...∗100+1=250∗(1∗2∗3∗..∗50)+1=250∗50!+1

Now, two numbers h(100)=250∗50!h(100)=250∗50! and h(100)+1=250∗50!+1h(100)+1=250∗50!+1 are consecutive integers. Two consecutive integers are co-prime, which means that they don't share ANY common factor but 1. For example 20 and 21 are consecutive integers, thus only common factor they share is 1.

As h(100)=250∗50!h(100)=250∗50! has all prime numbers from 1 to 50 as its factors, according to above h(100)+1=250∗50!+1h(100)+1=250∗50!+1 won't have ANY prime factor from 1 to 50. Hence pp (>1>1), the smallest prime factor of h(100)+1h(100)+1 will be more than 50.

Answer: E.

http://gmatclub.com/forum/what-is-smallest-prime-factor-of-h-100-1-where-h-n-is-212521.html

要点：两个连续自然数的公因数只有1。

hahouhei · 发表于 2016-9-15 19:59:38

of the 60 animals on a farm,2/3 are either pigs or cows,how many of the animals are cows?
(1)the farm has more than twice as many cows as it has pigs
(2)the farm has more than 12 pigs
statement(1)alone is sufficient,but (2)alone is not sufficient(my choice)
both statements together are sufficient,but neither alone is sufficient.(correct answer)

要点是倍数的表达，the farm has more than twice as many cows as it has pigs 牛是猪的两倍多

hahouhei · 发表于 2016-9-15 20:10:06

What is the hundredths digit of the decimal z ?

(1) The tenths digit of 100z is 2.
(2) The units digit of 1,000z is 2.

两个条件的意思都是z的千分位是2

hahouhei · 发表于 2016-9-15 20:16:46

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.

这道题题目是问你P(A+B) = P(A) + P(B) - P(AB)
1P(AB)=0是不充分的.因为如果同时发生的概率为0那么说明白球上都不是偶数,但是不代表不是偶数的都是白球.所以不充分
所以锁定选项B/C/E
2白球的概率-偶数概率为0.2,P(A) - P(B) = 0.2 .无法解出 P(A) + P(B)
所以两个加起来也不充分

http://www.zybang.com/question/02b5513c8cb5d3d3ff640b455a50592b.html

hahouhei · 发表于 2016-9-15 20:21:56

A certain city with a population of 132,000 is to be divided into 11 voting districts,and no district is to have a population that is more than 10 percent greater than the population of any other district.What is the minimum possible population that the least populated district could have?

总共11个区,要使最小区人数呈最小值,则其他10个区人数均呈最大值.
设最小区的人数为 x ,即为:
x + 10(1+10%) x = 132,000
x=1,100

http://www.zybang.com/question/e1f5e27f123ae332e3ce4f0cc1dd1dbe.html

hahouhei · 发表于 2016-9-17 20:15:29

Inuits of the Bering Sea were in isolation from contact with Europeans longer than Aleuts or Inuits of the North Pacific and northern Alaska.
(A) in isolation from contact with Europeans longer than
(B) isolated from contact with Europeans longer than
(C) in isolation from contact with Europeans longer than were
(D) isolated from contact with Europeans longer than were
(E) in isolation and without contacts with Europeans longer than

this is (d).

* "isolated" is plainly better than "in isolation", so we can narrow the problem to (b) and (d) right away.
and if you entertained the thought of choosing (e) even for two seconds, you really need to read through a bunch of correct answers - just read through the correct answers - to get a handle for what the correct sort of english looks like on these things. "in isolation and without contacts" is not only laughably wordy, but also redundant (it says exactly the same thing twice).

before we look at the actual difference between (b) and (d), a general takeaway:
* if you have 2 ANSWER CHOICES THAT DIFFER ONLY IN THE PRESENCE/ABSENCE OF A LITTLE WORD, then CHANCES ARE THAT THE LITTLE WORD IS NECESSARY. i can't guarantee this 100%, but LOOK FOR AN AMBIGUITY that arises if the little word is omitted.
* if the little word isn't the _sole_ difference between 2 answer choices, then try to concentrate on the other differences between the answer choices first.

in this problem, (b) is ambiguous. it could mean one of the following 2 things: ("BI" = bering inuits, "E" = europeans, "A" = aleuts etc.)
* BI were isolated from E longer than BI were isolated from A
* BI were isolated from E longer than A were isolated from E
we can't tell.
if you add the "were", then the meaning is narrowed down to the latter of these two possibilities.

--

analogy:
i know more about shakespeare than my brother
ambiguous.
two ways we can resolve this problem by adding little words:
1) i know more about shakespeare than does my brother --> i'm a bigger shakespeare nerd than my brother is.
2) i know more about shakespeare than about my brother --> my brother and i were separated at birth, so i know more about shakespeare than i do about him.
same deal.

https://www.manhattanprep.com/gmat/forums/inuits-of-the-bering-sea-were-in-isolation-from-contact-with-t4217.html

hahouhei · 发表于 2016-9-17 20:27:58

According to a study by the Carnegie Foundation for the Advancement of Teaching, companies in the United States are providing job training and general education for nearly eight million people, about equivalent to the enrollment of the nation’s four-year colleges and universities.

(A) equivalent to the enrollment of
(B) the equivalent of those enrolled in
(C) equal to those who are enrolled in
(D) as many as the enrollment of
(E) as many as are enrolled in

E Correct. In this sentence, as many as compares countable quantities. People are compared with people, the understood subject of are enrolled.

This sentence compares two groups of people. The best phrase to use for a comparison of two countable quantities (such as people) is as many as. The number of people in job training should be compared to the number of people in colleges, but the original sentence mistakenly compares people to enrollment.
----
Comparisons in SC must compare things that are structurally similar (grammatically parallel) and logically comparable (it must make sense to compare them). A lot of comparison questions will depend on the meaning - WHAT is being compared?

The comparison we're dealing with here is: ... eighty million people, about [the same number as another group of people]. We need to make sure that we're comparing numbers of people to numbers of people. As others have pointed out, most answer choices have an illogical comparison. Just to sum it all up in one post...

(A) equivalent to the enrollment of
Here we're comparing "80 million people" to "the enrollment." Not comparable.

(B) the equivalent of those enrolled in
"Those" stands in for "those people enrolled..." This means that we're comparing a number of people (80 million people) to simply "people." Not comparable.

(C) equal to those who are enrolled in
Same problem as B. Also, "equal" is qualitative, not quantitative. We don't want to say that they're just as good as other people, we want to say that there are just as many people.

(D) as many as the enrollment of
Again, we're comparing a number of people to "enrollment." Changing the comparison marker to "as many as" doesn't fix this.

(E) as many as are enrolled in
The ARE in this case implies that the first thing we were comparing is now the subject of this verb. In other words, this is effectively saying "as many as [the number of people who] are enrolled." This is the only one that compares number of people to number of people. CORRECT.

The answer is E.

http://www.beatthegmat.com/according-to-a-study-by-the-carnegie-foundation-t10203.html

hahouhei · 发表于 2016-9-18 11:52:36

There is a great deal of geographical variation in the frequency of many surgical procedures—up to tenfold variation per hundred thousand between different areas in the numbers of hysterectomies, prostatectomies, and tonsillectomies.

To support a conclusion that much of the variation is due to unnecessary surgical procedures, it would be most important to establish which of the following?

(A) A local board of review at each hospital examines the records of every operation to determine whether the surgical procedure was necessary.

(B) The variation is unrelated to factors (other than the surgical procedures themselves) that influence the incidence of diseases for which surgery might be considered.

(C) There are several categories of surgical procedure (other than hysterectomies, prostatectomies, and tonsillectomies) that are often performed unnecessarily.

(D) For certain surgical procedures, it is difficult to determine after the operation whether the procedures were necessary or whether alternative treatment would have succeeded.

(E) With respect to how often they are performed unnecessarily, hysterectomies,

prostatectomies, and tonsillectomies are representative of surgical procedures in general.

做这道题的时候，我还是按照以前总结的顺序问题=〉原文=〉问题=〉选项。但是当我看完原文开始再看问题时，头脑一片空白。这是我今天早上半个小时18题中的第一道，当时心都凉了。不用说，这道题当然做错了。这也是我做过得最令人费解的CR之一。

费解的原因，分析一下，主要有：

1。文章比较短，信息比较少。

2。生词比较多，看了就怕。

3。话题不熟悉，没有亲切感。

4。逻辑很晦涩，找不到北。

总之，我看了这道题以后彻底晕倒。混乱之中，选了C。当然，是错误的选项。

看了OG的解释，不明白，于是到CD上来，翻了以前的帖子，发现讨论这题的人不多，可能这道题对大多数人来说不是难题，而恰巧，又是我的盲点。

仔细分析一下此题。“There is a great deal of geographical variation in the frequency of many surgical procedures”是原文的中心句，也是结论。第一遍阅读大多数人，像我，都可能不明白，于是继续看。破折号后面的应该是解释或者举例。在破折号后面的句子中虽说有很多生词，但大致的意思就是说不同地方(between different areas)的差别(variation)很大，其中，作者还引入了数字来证明。OK，既然后面的是解释和举例，那么我们还是将重点放到第一句话。至此，我们应该能大概理解，第一句话是说“地理因素差异存在于外科手术当中”，也就是说，地理因素的不同导致了很多外科手术的不同。（注：这里Frequency是什么意思暂且不论）

了解了原文大概的意思，我们回到问题。“To support a conclusion that much of the variation is due to unnecessary surgical procedures, it would be most important to establish which of the following?” 在这里，我们可以发现，在问题当中，作者提出了结论，并且让我们结合原文，推出什么选项能support这个结论。在这里，结论是“much of the variation is due to unnecessary surgical procedures”。看看结论，我们可以发现，作者的想法是这些variation是由于很多不需要的procedures导致的。粗浅的分析一下，要证明这种想法，可能的方法就是证明variation不是由于其他的原因导致的。

现在，在分析完原文和问题后，我们看选项。

(A) A local board of review at each hospital examines the records of every operation to determine whether the surgical procedure was necessary. ==> A选项提出的事实是每个医院的local董事会通过审查每个手术的记录来决定手术流程是否必要。显然，这提出了另外一种variation的原因，所以是错误选项。

(B) The variation is unrelated to factors (other than the surgical procedures themselves) that influence the incidence of diseases for which surgery might be considered. ==> B选项指出Variation，也就是不同的手术流程，并不是由于疾病不同而产生的。也就是说，B选项排除了variation产生的一些可能性。从另一方面，这种分析促进了conclusion。所以正确。

(C) There are several categories of surgical procedure (other than hysterectomies, prostatectomies, and tonsillectomies) that are often performed unnecessarily. ==> C选项提出一些事实有很多不必要的procedures.咋看起来，好像正确，但其实，conclusion是说variation是由于unnecessary procedures。所以C选项偏离了结论的意思。

(D) For certain surgical procedures, it is difficult to determine after the operation whether the procedures were necessary or whether alternative treatment would have succeeded. ==> 对procedure是否necessary的讨论偏离了结论。

(E) With respect to how often they are performed unnecessarily, hysterectomies,

prostatectomies, and tonsillectomies are representative of surgical procedures in general. ==> E选项偏得更远。

http://forum.chasedream.com/thread-169083-1-1.html

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