Let # denote a mathematical operation. Is it true that x#y = y#x for all x and y?
(1) x#y=(1/x)+(1/y)
(2) x#y=x-y
My answer:
(1) YES, because commutative property of addition: (1/x)+(1/y) = (1/y)+(1/x)
(2) NO, because x-y not equal to y-x. But what if we take the number 0; then x-y=y-x (0-0=0-0). So (2) is not sufficient and answer could be A
The book (NOVA's GMAT Prep Course) says that the answer is D.
Can anyone please help?
Symbols-problem question
This topic has expert replies
-
- Master | Next Rank: 500 Posts
- Posts: 271
- Joined: Tue May 22, 2012 3:22 am
- Thanked: 7 times
- Followed by:3 members
Hi Anand,
Statement 1:
Your reasoning is correct, as statement one is sufficient due to commutative property of addition.
Statement 2 can be analysed putting value X=5,Y=3
X-Y=2
Y-X=-2
So here x#y not = y#x for all X and Y.
Hence it answers the question( Is it true that x#y = y#x for all x and y?)Answer is No.
Remember this is Yes-No question of data sufficiency.You only need to definitively answers yes or no without doubt.
So answer is D(both statements are sufficient alone)
----------------------------
Hope you like the answer,if it helped take a sec to click the "thank" button
Manpreet
Statement 1:
Your reasoning is correct, as statement one is sufficient due to commutative property of addition.
Statement 2 can be analysed putting value X=5,Y=3
X-Y=2
Y-X=-2
So here x#y not = y#x for all X and Y.
Hence it answers the question( Is it true that x#y = y#x for all x and y?)Answer is No.
Remember this is Yes-No question of data sufficiency.You only need to definitively answers yes or no without doubt.
So answer is D(both statements are sufficient alone)
----------------------------
Hope you like the answer,if it helped take a sec to click the "thank" button
Manpreet
The question that is asked is: Is it true that x#y = y#x for all x and y?anand108 wrote:Right. So my question is: what if x=y=0? Then the (2)becomes inconsistent, doesn't it?
If x#y=x-y, then does x#y = y#x for all x and y?
manpreet singh is correct...we can answer definitively that it is not true for all x and y. It doesn't matter whether it's true for x=y=0. We want to know whether it is true for all x and y, and it is not.
I came back and edited this answer after I saw how it was written.
The red text above is written ambiguously. The opposite of 'x#y = y#x is true for all x and y' is 'there exists x and y that x#y = y#x is false'.
GMAT/MBA Expert
- Anurag@Gurome
- GMAT Instructor
- Posts: 3835
- Joined: Fri Apr 02, 2010 10:00 pm
- Location: Milpitas, CA
- Thanked: 1854 times
- Followed by:523 members
- GMAT Score:770
Statement 1: x#y = (1/x) + (1/y) = (1/y) + (1/x) = y#xanand108 wrote:Let # denote a mathematical operation. Is it true that x#y = y#x for all x and y?
(1) x#y=(1/x)+(1/y)
(2) x#y=x-y
Hence, it is true that for all x and y, x#y = y#x.
Hence, answer to the question is YES.
Sufficient
Statement 2: x#y = (x - y) and y#x = (y - x)
If x = y = 0, then x#y = y#x
Otherwise, x#y ≠y#x
Hence, it is not true that for all x and y, x#y = y#x.
Hence, answer to the question is NO.
Sufficient
The correct answer is D.
Note : This is not a proper GMAT DS question. In GMAT you will never find a DS question in whih two statements leads to different answers to the original question.
Anurag Mairal, Ph.D., MBA
GMAT Expert, Admissions and Career Guidance
Gurome, Inc.
1-800-566-4043 (USA)
Join Our Facebook Groups
GMAT with Gurome
https://www.facebook.com/groups/272466352793633/
Admissions with Gurome
https://www.facebook.com/groups/461459690536574/
Career Advising with Gurome
https://www.facebook.com/groups/360435787349781/
GMAT Expert, Admissions and Career Guidance
Gurome, Inc.
1-800-566-4043 (USA)
Join Our Facebook Groups
GMAT with Gurome
https://www.facebook.com/groups/272466352793633/
Admissions with Gurome
https://www.facebook.com/groups/461459690536574/
Career Advising with Gurome
https://www.facebook.com/groups/360435787349781/