BREAKING: Target Test Prep releases Brand New 2026 On Demand GMAT prep course

Redeem

OG Quant Review question - I don't quite get this

This topic has expert replies
Post new topic Post Reply
Previous Topic Next Topic
Junior | Next Rank: 30 Posts
Posts: 17
Joined: Mon Sep 29, 2008 12:24 pm
Thanked: 1 times

OG Quant Review question - I don't quite get this

by ab78 » Tue Oct 28, 2008 3:50 am
If xy > 0 , does (x-1)(y-1) = 1 ?

1. x+y = xy
2. x=y

OA = A

I got it as D and I can explain why. Don't know if I am doing some stupid mistake.

1 . Is sufficient pretty straightforward.
2. (y-1)(y-1)=1 thus y^2 - 2y=0 thus we get either y=0 or y=2. since they have mentioned xy>0 y can't be zero, it has to be 2 and hence x=2 and I am getting Sufficient answer. If the OA in OG is correct then I am really doing some real stupid mistake.

Can someone point out what mistake I am doing?
[/spoiler]
Top
Source: — Data Sufficiency |
User avatar
Legendary Member
Posts: 2134
Joined: Mon Oct 20, 2008 11:26 pm
Thanked: 237 times
Followed by:25 members
GMAT Score:730

by logitech » Tue Oct 28, 2008 7:22 am
AB78,

For example:

For example:

Most of the people forget that an equation such as X^2=2X has 2 solutions!

X^2 - 2X = 0

x( X- 2) = 0

SO X = 0 and X = 2 ( People usually forget about X )

Going back to our problem:

(x-1)(y-1) = 1 can be simplified as X+y=xy as in Statement (1)

if you try to find X from this equation:

xy-x=y

x(y-1)=y

X = Y / Y-1

So first of all, you can see that Y can't be equal to 1! so x=y=1 is not s solution for this equation.

But x=y=2 is

INSUF

so it is A.

Hope it helps!

:idea:
LGTCH
---------------------
"DON'T LET ANYONE STEAL YOUR DREAM!"
Top
Newbie | Next Rank: 10 Posts
Posts: 1
Joined: Thu Oct 23, 2008 12:38 pm

by rock1981 » Tue Oct 28, 2008 10:47 am
I would say D is the answer. Please post if you find out why B would be insufficient.

Thanks.
Top
User avatar
GMAT Instructor
Posts: 3225
Joined: Tue Jan 08, 2008 2:40 pm
Location: Toronto
Thanked: 1710 times
Followed by:614 members
GMAT Score:800

Re: OG Quant Review question - I don't quite get this

by Stuart@KaplanGMAT » Tue Oct 28, 2008 11:14 am
ab78 wrote:If xy > 0 , does (x-1)(y-1) = 1 ?

1. x+y = xy
2. x=y

OA = A

I got it as D and I can explain why. Don't know if I am doing some stupid mistake.

1 . Is sufficient pretty straightforward.
2. (y-1)(y-1)=1 thus y^2 - 2y=0 thus we get either y=0 or y=2. since they have mentioned xy>0 y can't be zero, it has to be 2 and hence x=2 and I am getting Sufficient answer. If the OA in OG is correct then I am really doing some real stupid mistake.

Can someone point out what mistake I am doing?
[/spoiler]
Picking numbers clearly illustrates that (2) isn't sufficient.

Does (x-1)(y-1) = 1?

(2) x = y

if we pick x=y=2, then we get the question:

Does 1*1 = 1? YES

if we pick x = y = 3, then we get the question:

Does 2*2 = 1? NO

The mistake you made was that you didn't try to answer the question - you assumed that (x-1)(y-1) = 1 is a fact, which is not the case.

Remember, the STATEMENTS are facts that we take to be true. The QUESTION is what we're trying to figure out.

So, when dealing with (2) algebraically, we get:

Is (y-1)(y-1) = 1?

Is y^2 - 2y - 1 = 1?

Is y^2 - 2y = 0?

Is y(y-2) = 0?

Well, if y=0 or y=2, then YES it is (and as you pointed out, y can't = 0, but it still could = 2).

However, if y is any other number, then NO it isn't.

So, the answer to the question could be YES or NO: insufficient.
Image

Stuart Kovinsky | Kaplan GMAT Faculty | Toronto

Kaplan Exclusive: The Official Test Day Experience | Ready to Take a Free Practice Test? | Kaplan/Beat the GMAT Member Discount
BTG100 for $100 off a full course
Top
User avatar
Legendary Member
Posts: 2134
Joined: Mon Oct 20, 2008 11:26 pm
Thanked: 237 times
Followed by:25 members
GMAT Score:730

by logitech » Tue Oct 28, 2008 11:15 am
rock1981 wrote:I would say D is the answer. Please post if you find out why B would be insufficient.

Thanks.
(x-1)(y-1) = 1 ?

What happens when you have X=Y=1

Will you get 1 ? or 0 ?
LGTCH
---------------------
"DON'T LET ANYONE STEAL YOUR DREAM!"
Top
Junior | Next Rank: 30 Posts
Posts: 17
Joined: Mon Sep 29, 2008 12:24 pm
Thanked: 1 times

by ab78 » Wed Oct 29, 2008 5:33 am
Thanks everyone for the reply. I appreciate the responses and I now know where I went wrong.
Top
Junior | Next Rank: 30 Posts
Posts: 17
Joined: Mon Sep 29, 2008 12:24 pm
Thanked: 1 times

by ab78 » Wed Oct 29, 2008 5:35 am
Logitech on different note your signature is pretty interesting :)
Top
Legendary Member
Posts: 2467
Joined: Thu Aug 28, 2008 6:14 pm
Thanked: 331 times
Followed by:11 members

by cramya » Wed Oct 29, 2008 8:37 pm
A)

If xy > 0 , does (x-1)(y-1) = 1 ?

1. x+y = xy


Simplify (x-1)(y-1) = 1
xy-x-y+1=1
xy-x-y=0

Stmt I) is exactly this

xy-x-y=0 No proof necessary
Top
Newbie | Next Rank: 10 Posts
Posts: 6
Joined: Wed Nov 05, 2008 4:56 pm
Location: Plano Texas

Thanks. Great Answer.

by jmdgirish » Tue Dec 30, 2008 8:38 pm
Awesome Answer and a such a a painfully obvious thing pointed out.

Great. Thanks.
- Regards,

Girish Malik
Top
User avatar
Legendary Member
Posts: 546
Joined: Sun Nov 16, 2008 11:00 pm
Location: New Delhi , India
Thanked: 13 times

Re: OG Quant Review question - I don't quite get this

by ronniecoleman » Wed Dec 31, 2008 12:46 am
ab78 wrote:If xy > 0 , does (x-1)(y-1) = 1 ?

1. x+y = xy
2. x=y

OA = A

I got it as D and I can explain why. Don't know if I am doing some stupid mistake.

1 . Is sufficient pretty straightforward.
2. (y-1)(y-1)=1 thus y^2 - 2y=0 thus we get either y=0 or y=2. since they have mentioned xy>0 y can't be zero, it has to be 2 and hence x=2 and I am getting Sufficient answer. If the OA in OG is correct then I am really doing some real stupid mistake.

Can someone point out what mistake I am doing?
[/spoiler]
A very simple approach!!

given xy >0
so neither x nor y would be 0

1) x + y = xy

x( y-1) = y

since x can't be 0
so y-1 = y/x

on same lines x-1 = x/y

hence (x-1)*(y-1) = 1

suff..


2) x = y

(x-1)^2 ?

we don't know the value of x

hence

insuff..


Logitech,

X = Y / Y-1

So first of all, you can see that Y can't be equal to 1! so x=y=1 is not s solution for this equation. ????
Admission champion, Hauz khaz
011-27565856
Top
Post new topic Post Reply
• Page 1 of 1