It is currently 18 Nov 2017, 20:36

GMAT Club Daily Prep

Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

Events & Promotions

Events & Promotions in June
Open Detailed Calendar

m23 #33

Author Message
Director
Joined: 12 Jul 2008
Posts: 514

Kudos [?]: 165 [2], given: 0

Schools: Wharton

Show Tags

15 Aug 2008, 09:22
2
KUDOS
4
This post was
BOOKMARKED
For any numbers $$x$$ and $$y$$ , $$x#y = xy - x - y$$ . If $$x#y = 1$$ , which of the following cannot be the value of y ?

(A) -2
(B) -1
(C) 0
(D) 1
(E) 2

[Reveal] Spoiler: OA
D

Source: GMAT Club Tests - hardest GMAT questions

OA is D. How can y = 0 if xy = 1?

Kudos [?]: 165 [2], given: 0

Intern
Joined: 11 Jan 2010
Posts: 38

Kudos [?]: 54 [14], given: 9

Show Tags

05 Feb 2010, 07:55
14
KUDOS
Given x # y = xy - x - y

If x # y = 1 => xy - x - y = 1

Solving for the value of x:
xy - x - y = 1
=> x (y - 1) = (y + 1)
=> x = (y + 1) / ( y - 1)

So, y - 1 cannot be 0; therefore, y cannot be 1.

=====================================================
Dear All: I'm looking for a study partner. I live in Plainsboro/Princeton-New Jersey.

Last edited by vshrivastava on 22 Feb 2013, 01:15, edited 1 time in total.

Kudos [?]: 54 [14], given: 9

Director
Joined: 12 Jul 2008
Posts: 514

Kudos [?]: 165 [2], given: 0

Schools: Wharton

Show Tags

15 Aug 2008, 11:37
2
KUDOS
jallenmorris wrote:
It's still a good question.

I plugged some numbers.

x =2, finds that y = 1

x = 3, finds that y = 2

so y = x+1

If y = 1, then x = 0 and you do not get 1 from x#y.

Another way to do it:

1 = xy - x - y
1 = x*(y-1) - y
y + 1 = x*(y-1)

If y = 1, this equation does not hold.

Kudos [?]: 165 [2], given: 0

Math Expert
Joined: 02 Sep 2009
Posts: 42249

Kudos [?]: 132626 [2], given: 12326

Show Tags

11 Jan 2013, 06:42
2
KUDOS
Expert's post
zoinnk wrote:
For any numbers $$x$$ and $$y$$ , $$x#y = xy - x - y$$ . If $$x#y = 1$$ , which of the following cannot be the value of y ?

(A) -2
(B) -1
(C) 0
(D) 1
(E) 2

[Reveal] Spoiler: OA
D

Source: GMAT Club Tests - hardest GMAT questions

OA is D. How can y = 0 if xy = 1?

For any numbers $$x$$ and $$y$$, $$x@y=xy-x-y$$. If $$x@y=1$$, which of the following cannot be the value of $$y$$ ?

A. -2
B. -1
C. 0
D. 1
E. 2

Given $$xy-x-y=1$$, which is the same as $$(1-x)(1-y)-1=1$$ or $$(1-x)(1-y)=2$$. Now, if $$y=1$$ then $$(1-x)(1-1)=0\neq{2}$$, so in order the given equation to hold true $$y$$ cannot equal to 1.

_________________

Kudos [?]: 132626 [2], given: 12326

SVP
Joined: 30 Apr 2008
Posts: 1863

Kudos [?]: 623 [1], given: 32

Location: Oklahoma City
Schools: Hard Knocks

Show Tags

15 Aug 2008, 10:02
1
KUDOS
It's still a good question.

I plugged some numbers.

x =2, finds that y = 1

x = 3, finds that y = 2

so y = x+1

If y = 1, then x = 0 and you do not get 1 from x#y.
_________________

------------------------------------
J Allen Morris
**I'm pretty sure I'm right, but then again, I'm just a guy with his head up his a.

GMAT Club Premium Membership - big benefits and savings

Kudos [?]: 623 [1], given: 32

Intern
Joined: 02 Nov 2009
Posts: 20

Kudos [?]: 76 [1], given: 9

Show Tags

05 Feb 2010, 09:41
1
KUDOS
For any numbers x and y , x#y = xy - x - y . If x#y = 1 , which of the following cannot be the value of y ?

(C) 2008 GMAT Club - m23#33

A -2
B -1
C 0
D 1
E 2

My way:
xy-x-y=1
-> x(y-1)-y=1
->x(y-1)=1+y
->x=(1+y)/(y-1)
-----> y-1 should be 0
so y cannot be 1.

Kudos [?]: 76 [1], given: 9

CEO
Status: Nothing comes easy: neither do I want.
Joined: 12 Oct 2009
Posts: 2757

Kudos [?]: 1909 [1], given: 235

Location: Malaysia
Concentration: Technology, Entrepreneurship
Schools: ISB '15 (M)
GMAT 1: 670 Q49 V31
GMAT 2: 710 Q50 V35

Show Tags

05 Feb 2010, 13:35
1
KUDOS
IMO D

Another way

xy-x-y = 1

=> y = $$\frac{(x+1)}{(x-1)}$$

clearly y =1 is not possible as x+1 can never be equal to x-1
_________________

Fight for your dreams :For all those who fear from Verbal- lets give it a fight

Money Saved is the Money Earned

Jo Bole So Nihaal , Sat Shri Akaal

GMAT Club Premium Membership - big benefits and savings

Gmat test review :
http://gmatclub.com/forum/670-to-710-a-long-journey-without-destination-still-happy-141642.html

Kudos [?]: 1909 [1], given: 235

Manager
Joined: 05 Dec 2009
Posts: 126

Kudos [?]: 87 [1], given: 0

Show Tags

13 Mar 2010, 17:53
1
KUDOS
xy-x-y = 1 and if y=1, the equation will result in -1 = +1...not possible...so Ans is D.

Kudos [?]: 87 [1], given: 0

Manager
Joined: 20 Oct 2013
Posts: 75

Kudos [?]: 12 [1], given: 15

Location: United States
Concentration: General Management, Real Estate

Show Tags

21 Apr 2014, 05:45
1
KUDOS
Plug in the numbers and see how:
A) -2x-x+2=1->x=1/3
B) -x-x+1=1-> x=0
C) -x=1
D) x-x-1=1=> -1=1 (ten ten!)

So choose D

Kudos [?]: 12 [1], given: 15

Manager
Joined: 23 Nov 2009
Posts: 89

Kudos [?]: 43 [0], given: 14

Schools: Wharton..:)

Show Tags

05 Feb 2010, 08:17
pretty easy ::
xy-(x+y)=1

from options::
put y=1
x-(1+x)=1
to satisfy this...
-1=1 not possible
so ans is for y=1
-----------------
rest all the values would give sm ans ..
--------------------------
_________________

" What [i] do is not beyond anybody else's competence"- warren buffett
My Gmat experience -http://gmatclub.com/forum/gmat-710-q-47-v-41-tips-for-non-natives-107086.html

Kudos [?]: 43 [0], given: 14

Intern
Joined: 31 Jan 2010
Posts: 6

Kudos [?]: 28 [0], given: 1

Show Tags

08 Feb 2010, 15:22
x#y =1 => xy-x-y =1
=> x(y-1) = 1+y
=> x = (1+y)/(y-1)

denominator cannot be zero hence y != 1

Kudos [?]: 28 [0], given: 1

Manager
Joined: 13 Dec 2009
Posts: 248

Kudos [?]: 256 [0], given: 13

Show Tags

19 Mar 2010, 08:23
zoinnk wrote:
For any numbers $$x$$ and $$y$$ , $$x#y = xy - x - y$$ . If $$x#y = 1$$ , which of the following cannot be the value of y ?

(A) -2
(B) -1
(C) 0
(D) 1
(E) 2

[Reveal] Spoiler: OA
D

Source: GMAT Club Tests - hardest GMAT questions

OA is D. How can y = 0 if xy = 1?

xy-x-y =1 => x(y-1) = 1+y => x = (1+y)/(y-1)
definitely y cannot 1 as this will make x indefinite.
so d
_________________

My debrief: done-and-dusted-730-q49-v40

Kudos [?]: 256 [0], given: 13

Intern
Status: Preparing again for second attempt....
Joined: 11 Dec 2010
Posts: 24

Kudos [?]: 9 [0], given: 2

WE 1: 6 years

Show Tags

10 Feb 2011, 06:23
Just plug in the answer choices to find the correct answer. Kaplan teaches us to try options D and B first because 60% of the times, the correct answer is either of them. (esp true for questions that can be solved by plugging values from answers choices)

xy - x - y = 1
xy - x = 1 + y

From D, y=1
x.1 - x = 1 + 1
x - x = 2
This cannot be true for no matter what value x takes, hence we have our correct answer. (luckily without looking at other options -- thanks Kaplan)

Kudos [?]: 9 [0], given: 2

Senior Manager
Joined: 01 Nov 2010
Posts: 282

Kudos [?]: 87 [0], given: 44

Location: India
Concentration: Technology, Marketing
GMAT Date: 08-27-2012
GPA: 3.8
WE: Marketing (Manufacturing)

Show Tags

10 Feb 2011, 10:58
another way of finding the answer;
check all the options and eliminate the odd one.

option 1. y=-2
then, x#y=1=xy-x-y
x=1/3
option 2.y=-1
then, x#y=1=xy-x-y
x=0
option 3.y=0
then, x#y=1=xy-x-y
x=-1
option 4.y=1
then, x#y=1=xy-x-y ==>1=-1, which is wrong
so, D cant be an option.
option 5,y=2
then, x#y=1=xy-x-y
x=3

hence option D is correct.
_________________

kudos me if you like my post.

Attitude determine everything.
all the best and God bless you.

Kudos [?]: 87 [0], given: 44

Intern
Joined: 23 Nov 2010
Posts: 1

Kudos [?]: [0], given: 0

Show Tags

14 Feb 2011, 09:55
@all
I think we all are overlooking a fact out here that in the question its given ,
X#Y = 1 ,
how i approached this questn was to find out a value of X by putting in values of Y from the options , taking into account a feasible operator, and then putting the values of Both X & Y in the second eqn to see if it results in 1

for say if Y = 0 , X has to be +1 and #(operator) should be ADDITION /SUBTRACTION ,
but if now we put X= 1 and Y= 0 in xy-x-y = 1 , it would give -1 = 1 , thus Y != 0

Kudos [?]: [0], given: 0

Math Expert
Joined: 02 Sep 2009
Posts: 42249

Kudos [?]: 132626 [0], given: 12326

Show Tags

14 Feb 2012, 07:53
raulsy wrote:
@all
I think we all are overlooking a fact out here that in the question its given ,
X#Y = 1 ,
how i approached this questn was to find out a value of X by putting in values of Y from the options , taking into account a feasible operator, and then putting the values of Both X & Y in the second eqn to see if it results in 1

for say if Y = 0 , X has to be +1 and #(operator) should be ADDITION /SUBTRACTION ,
but if now we put X= 1 and Y= 0 in xy-x-y = 1 , it would give -1 = 1 , thus Y != 0

Welcome to GMAT Club.

The point is that # represents some functional relationship between $$x$$ and $$y$$ described as $$x#y=xy-x-y$$. So # does not represent any arithmetic operation: +, -, /, or *.

Hope it's clear.
_________________

Kudos [?]: 132626 [0], given: 12326

Intern
Joined: 11 Jul 2012
Posts: 12

Kudos [?]: 12 [0], given: 21

Show Tags

11 Jan 2013, 06:38
zoinnk wrote:
For any numbers $$x$$ and $$y$$ , $$x#y = xy - x - y$$ . If $$x#y = 1$$ , which of the following cannot be the value of y ?

(A) -2
(B) -1
(C) 0
(D) 1
(E) 2

[Reveal] Spoiler: OA
D

Source: GMAT Club Tests - hardest GMAT questions

OA is D. How can y = 0 if xy = 1?

Racked brains and got to D, only to find B marked wrongly in the inmail question of the day. PFA screenshot. Good question.
Attachments

File comment: Screenshot!

Untitled.png [ 14.66 KiB | Viewed 5049 times ]

_________________

all you need is a will.

Kudos [?]: 12 [0], given: 21

Intern
Joined: 11 Jul 2012
Posts: 12

Kudos [?]: 12 [0], given: 21

Show Tags

11 Jan 2013, 06:46
Bunuel wrote:
zoinnk wrote:
For any numbers $$x$$ and $$y$$ , $$x#y = xy - x - y$$ . If $$x#y = 1$$ , which of the following cannot be the value of y ?

(A) -2
(B) -1
(C) 0
(D) 1
(E) 2

[Reveal] Spoiler: OA
D

Source: GMAT Club Tests - hardest GMAT questions

OA is D. How can y = 0 if xy = 1?

For any numbers $$x$$ and $$y$$, $$x@y=xy-x-y$$. If $$x@y=1$$, which of the following cannot be the value of $$y$$ ?

A. -2
B. -1
C. 0
D. 1
E. 2

Given $$xy-x-y=1$$, which is the same as $$(1-x)(1-y)-1=1$$ or $$(1-x)(1-y)=2$$. Now, if $$y=1$$ then $$(1-x)(1-1)=0\neq{2}$$, so in order the given equation to hold true $$y$$ cannot equal to 1.

Bunuel! Great work.
_________________

all you need is a will.

Kudos [?]: 12 [0], given: 21

Math Expert
Joined: 02 Sep 2009
Posts: 42249

Kudos [?]: 132626 [0], given: 12326

Show Tags

13 Jan 2013, 04:14
californiaNY wrote:
since I posted that screenshot, my daily GMAT questions have stopped! Am I assuming things or are we getting a bug check?

Question of the day is NOT sent on Saturday's and Sunday's. You'll receive it tomorrow on Monday. Pleas PM me if you won't.
_________________

Kudos [?]: 132626 [0], given: 12326

Intern
Joined: 11 Sep 2013
Posts: 22

Kudos [?]: [0], given: 10

Location: India
GMAT 1: 660 Q48 V33

Show Tags

01 Feb 2014, 02:03
By plugging we can get that y cannot be equal to 1

Kudos [?]: [0], given: 10

Re: m23 #33   [#permalink] 01 Feb 2014, 02:03
Display posts from previous: Sort by

m23 #33

Moderator: Bunuel

 Powered by phpBB © phpBB Group | Emoji artwork provided by EmojiOne Kindly note that the GMAT® test is a registered trademark of the Graduate Management Admission Council®, and this site has neither been reviewed nor endorsed by GMAC®.