Author 
Message 
SVP
Joined: 03 Jan 2005
Posts: 2233

I'm starting a new thread to collect some traps that are [#permalink]
Show Tags
12 Oct 2006, 20:12
1
This post received KUDOS
2
This post was BOOKMARKED
This topic is locked. If you want to discuss this question please repost it in the respective forum. I'm starting a new thread to collect some traps that are commonly seen in GMAT. Please try to form your own answer to each question before you highlight the spoiler to see the official answer. You could also find more discussions regarding each question by searching the forum to find the original threads.
Please feel free to add to the thread when you discover a trap.
Hong
iced_tea wrote: Is integer x positive ?
1) x > x^3 2) x < x^2 [spoiler] OA: A, trap: "integer"[/spoiler] kevincan wrote: A triangle has sides of length 12, 15 and k units. What is the length of the longest side?
(1) The perimeter of the triangle is a multiple of 9. (2) One of the interior angles of the triangle is equal to the sum of the other two interior angles. [spoiler] OA: C, trap: special triangle[/spoiler] haas_mba07 wrote: If x is to be selected at random from set T, what is the probability that x/4 5 < 0?
(1). T is a set of 8 integers (2). T is contained in the set o fintegers from 1 to 25, inclusive. [spoiler] OA: E, trap: T is contained in a set[/spoiler] gmatornot wrote: If A is a prime number, what is the value of A? (1) 0 < A <10>0 but A2 can be 0[/color][/spoiler] u2lover wrote: Is 25/(a+1) an integer?
(1) a is a multiple of 25 (2) 0<=a<=25 [spoiler] OA: E, trap: 0 is a multiple of any umber[/spoiler] n_sathya wrote: In a class, 20 students speak spanish, 15 speak french and 15 speak chinese. how many students are in the class? 1. exactly 30% of the students in the class speak french 2. all of the students speak english and none speaks more than two languages.
[spoiler] OA: A, trap: You can't add students who speak Spanish, French and Chinese to get the number of students in the class. There might be students who speak Japanese and English, or any other languages. There could also be students who can only speak English.[/spoiler]
_________________
Keep on asking, and it will be given you;
keep on seeking, and you will find;
keep on knocking, and it will be opened to you.
Last edited by HongHu on 21 Dec 2006, 13:54, edited 5 times in total.



Manager
Status: Post MBA, working in the area of Development Finance
Joined: 09 Oct 2006
Posts: 169
Location: Africa

Is integer x positive ?
1) x > x^3
2) x < x^2
If x<0, only then x > x^3
If x>0, only then x < x^2
Aren't both the statements independently sufficient to answer?
What is amiss in my logic?



SVP
Joined: 01 May 2006
Posts: 1796

Good idea .... People will thus find in 1 topic all they need to remember.
Perhaps, it should be nice if we have a check list, a kind of XLS file, that contains some column such as Category/Problem/Soluion/link to a topic.



Intern
Joined: 27 Aug 2006
Posts: 32

1
This post received KUDOS
The ans is A .
The first statement alone is sufficient .
Here is the break down
1. X>X^3
If u take postive value of X , then above value is false .
If u take x=3 ,then x^3 = 27 , hence the given A is true and we know
X is not a positive integer.
2. X<X^2  now its almost a common knowldege tht whenever u see ^2 or X ,its a trap .
For example x=2 < X^2 = 4  So true
also if X=2 < 2^2 = 4
Hence insufficient .
So 1 alone is sufficient .
hope this helps



Manager
Joined: 01 Feb 2006
Posts: 78
Location: New York

Is integer x positive ?
1) x > x^3
2) x < x^2
1) For values, 1, 1,2 inequality is false. For 2 is true. SUF
2) For values 1, 1 inequality os false. True for 2, 2. INSUF
Answer A.



SVP
Joined: 03 Jan 2005
Posts: 2233

For people who may not aware, I've been collecting different questions and updating the opening post. In other words there are more than one question there now. I think the best way to go is to compare your answer with the spoiler. If it is correct then no need to post. However if you have a question please post it here by all means.
I like Fig's suggestion too. I'll try to do something like that when we have a decent collection of the traps.
_________________
Keep on asking, and it will be given you;
keep on seeking, and you will find;
keep on knocking, and it will be opened to you.



Manager
Status: Post MBA, working in the area of Development Finance
Joined: 09 Oct 2006
Posts: 169
Location: Africa

GmatInstinct wrote: The ans is A .
The first statement alone is sufficient . Here is the break down
1. X>X^3 If u take postive value of X , then above value is false . If u take x=3 ,then x^3 = 27 , hence the given A is true and we know X is not a positive integer.
2. X<X^2  now its almost a common knowldege tht whenever u see ^2 or X ,its a trap . For example x=2 < X^2 = 4  So true also if X=2 < 2^2 = 4
Hence insufficient .
So 1 alone is sufficient .
hope this helps
Thanks
I got trapped and for such silly lack of ratiocination. This is what happens when one is not fuuly attentive to the problem.
Thanks again.



Intern
Joined: 19 Oct 2006
Posts: 1

Answer is A [#permalink]
Show Tags
19 Oct 2006, 11:30
Have to be careful while solving inequalities:
1. x>x^3
=> xx^3>0
=>x(1x^2)>0
=>x(1x)(1+x)>0
x cannot be 0, 1 or +1 or the inequality does not work.
So if x=+2, then 2*1*3=6 which is NOT > 0 (the inequality is not true)
if x=2, then 2*3*1 =+6 which is >0. so x is ve
therefore x is negative.
2. x<x^2
=>xx^2<0
=>x(1x)<0
x cannot be o or 1
lets say x=+2, then inequality is true
lets say x=2, then x(1x)=2*3=6 <0 which is true.
In both cases (x being +ve and ve), the inequality holds true and so this choice doesnot help determine x.



Senior Manager
Joined: 20 Feb 2006
Posts: 373

Re: Math trap collection [#permalink]
Show Tags
08 Nov 2006, 13:02
kevincan wrote: A triangle has sides of length 12, 15 and k units. What is the length of the longest side?
(1) The perimeter of the triangle is a multiple of 9. (2) One of the interior angles of the triangle is equal to the sum of the other two interior angles.
[spoiler] OA: C, trap: special triangle[/spoiler]
Could someone explain this please?



Manager
Joined: 10 Jul 2006
Posts: 72

Triangle trap [#permalink]
Show Tags
08 Nov 2006, 18:37
From the stem and the triangle rules of 3 sides: 1512<k<15+12 or 3<k<27.
(1) Perimeter = 12+15+k = 27+k. P is also a multiple of 9, thus P must be number such as 27,36,45,54 etc. However, since k is limited to the range of 3 to 27 exclusive, P must either be 36 or 45. That means k is either 9 or 18. Insuff.
(2) This means the triangle is a right triangle. However, 15 does not need to be the hypotenus of the triangle. There are 2 cases.
* 15 is the hypotenus. Then we have the special triangle 91215.
* 15 is one of the leg. Then, using the theorem, the hypotenus k is about 19 (k^2 = 12^2 + 15^2= 369). Since k is still in the range provided in the stem. This is insuff to tell.
1+2: from 1: k = 9 or 18.
from 2: k = 9 or around 19. Thus, K must be 9 and the longest side is 15. Suff
Ans: C



Manager
Joined: 09 Nov 2006
Posts: 51

i think that (1) insuf
but for the second it cannot be concluded that two angels of the triangle are equal. It can be that y=z+x (all angles are different) from this we can conclude that x+y+z = 180 i.e. y+y=180, y=90 so there is right triangle. But this is not enough because either side can have 90degrees angle so we need first statement: 12^2+15^=k^2, or 15^12^2=k^2 and so on untill we will get the perimeter which will be the multiple of nine.
Am i right?



Intern
Joined: 11 Apr 2005
Posts: 11

When to consider zero in DS questions ? [#permalink]
Show Tags
18 Nov 2006, 11:27
Is 25/(a+1) an integer?
(1) a is a multiple of 25
(2) 0<=a<=25
[spoiler]OA: E, trap: 0 is a multiple of any number[/spoiler]
Would "0" be considered a multiple ?
I consider "0" when calculating for a number, integer, even number, to the power of (any number to the power of zero is 1, note 0 to the power 0 does not exist ?).
I do not consider "0" as a divisor (any nymer divided by does not exist), sq. root (should I consider it ??  example  sq.root of x = x), and also as a multiple.(should it be considered as a multiple?)
Where else would the zero trap come in ?
Thanks..



Senior Manager
Joined: 20 Feb 2006
Posts: 373

cute_smiley wrote: i think that (1) insuf
but for the second it cannot be concluded that two angles of the triangle are equal. It can be that y=z+x (all angles are different) from this we can conclude that x+y+z = 180 i.e. y+y=180, y=90 so there is right triangle. But this is not enough because either side can have 90degrees angle so we need first statement: 12^2+15^=k^2, or 15^12^2=k^2 and so on untill we will get the perimeter which will be the multiple of nine. Am i right?
I agree with you smiley  we cannot conclude that the two angels are the same but we do still need the first statement so I'm getting answer as C. However, when working through the problem I cant seem to get a suitable numerical answer.
We know that there is an angle of 90 degrees so if the triangle were an isosceles triangle then the sides would have to be in the ratio x: x: x*(sqrt2) which would not allow for sides of 12 and 15 or a perimeter with a multiple of 9.
From 1) we know the perimeter is a multiple of 9. The only configuration of sides with a 90 degree internal angle that would allow this is for the sides to be in the ratio 3:4:5 or 9:12:15  giving a perimeter of 36.
C is the answer
Last edited by MBAlad on 05 Dec 2006, 15:35, edited 2 times in total.



Manager
Joined: 18 Nov 2006
Posts: 123

make a silly mistake  here ! [#permalink]
Show Tags
04 Dec 2006, 12:29
If p,q,r are even numbers such that 2<p<q<r
what is the value of q ?
) r<10
2)p<6
guys, take as less time as possible while trying this and let me know the answer you are getting..



Manager
Joined: 25 Sep 2006
Posts: 151

Quote: A and B are different integer/numbers..... ........
Careful of the word different . Such condition tells a lot.
_________________
livin in a prison island...



Senior Manager
Joined: 24 Nov 2006
Posts: 349

Re: make a silly mistake  here ! [#permalink]
Show Tags
13 Dec 2006, 07:10
ItÂ´s A.
1): r<10 => r = 8 or 6 or 4, but as we also have p<q<r then r has to be 8, q = 6 and p = 4. ThereÂ´s no other set of numbers that fulfill the conditions stated so A is sufficient.
2): If p<6 then there are infinite values q can have, so B is not sufficient.
BLISSFUL wrote: If p,q,r are even numbers such that 2<p<q<r what is the value of q ? 1) r<10 2)p<6
guys, take as less time as possible while trying this and let me know the answer you are getting..



Manager
Joined: 04 Feb 2004
Posts: 70
Location: India

Re: make a silly mistake  here ! [#permalink]
Show Tags
21 Dec 2006, 10:25
Andr359 wrote: ItÂ´s A. 1): r<10 => r = 8 or 6 or 4, but as we also have p<q<r then r has to be 8, q = 6 and p = 4. ThereÂ´s no other set of numbers that fulfill the conditions stated so A is sufficient. 2): If p<6 then there are infinite values q can have, so B is not sufficient. BLISSFUL wrote: If p,q,r are even numbers such that 2<p<q<r what is the value of q ? 1) r<10 2)p<6
guys, take as less time as possible while trying this and let me know the answer you are getting..
Again, it does not say integer  the answer is E



Manager
Joined: 18 Nov 2006
Posts: 123

Re: make a silly mistake  here ! [#permalink]
Show Tags
21 Dec 2006, 11:55
deep, I think you are thinking too deep deeper than GMAT !??
I think even number means it refers to integer only.
Do we have even real numbers like 1.2,1.4 etc ??
whadya say folks ????

deep wrote: Andr359 wrote: ItÂ´s A. 1): r<10 => r = 8 or 6 or 4, but as we also have p<q<r then r has to be 8, q = 6 and p = 4. ThereÂ´s no other set of numbers that fulfill the conditions stated so A is sufficient. 2): If p<6 then there are infinite values q can have, so B is not sufficient. BLISSFUL wrote: If p,q,r are even numbers such that 2<p<q<r what is the value of q ? 1) r<10 2)p<6
guys, take as less time as possible while trying this and let me know the answer you are getting.. Again, it does not say integer  the answer is E



SVP
Joined: 03 Jan 2005
Posts: 2233

LOL yes I agree that even numbers have to be integers.
Another trap:
n_sathya wrote: In a class, 20 students speak spanish, 15 speak french and 15 speak chinese. how many students are in the class? 1. exactly 30% of the students in the class speak french 2. all of the students speak english and none speaks more than two languages.
[spoiler] OA: A, trap: You can't add students who speak Spanish, French and Chinese to get the number of students in the class. There might be students who speak Japanese and English, or any other languages. There could also be students who can only speak English.[/spoiler]
_________________
Keep on asking, and it will be given you;
keep on seeking, and you will find;
keep on knocking, and it will be opened to you.



Manager
Joined: 18 Nov 2006
Posts: 123

HongHu wrote: LOL yes I agree that even numbers have to be integers. Another trap: n_sathya wrote: In a class, 20 students speak spanish, 15 speak french and 15 speak chinese. how many students are in the class? 1. exactly 30% of the students in the class speak french 2. all of the students speak english and none speaks more than two languages.
[spoiler] OA: A, trap: You can't add students who speak Spanish, French and Chinese to get the number of students in the class. There might be students who speak Japanese and English, or any other languages. There could also be students who can only speak English.[/spoiler]

Yes HongHu,
This was a classic trap that we had solved recently..
I have read through your past postings; more than your scores, your story is amazing!!! Your presence is a huge plus to the community. I wish you all the best in what ever you do!!!!!







Go to page
1 2
Next
[ 28 posts ]



