OG - DS - #80

[GMAT 5/18]

....please stop the pain. :)

Source: GMAT Quant Review (Green Book) Official Guide, 11th Edition, Question 80, Data Sufficiency

I worked this problem, got the incorrect answer, reviewed the explanation, looked at my scribblings, and then came to the conclusion that the GMAC made a mistake. :) Then I thought, wait....the GMAC doesn't make mistakes, I do. So I repeated the steps above, and am still perplexed. So, if anyone can help, that would be awesome.

Here is the question and answers:

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

(1) x + y = xy
(2) x = y

Instead of writing my entire thoughts, I will just place 2 different values for x and y (that I feel satisfy both Statement I and II, as well as the question) - I believe these 2 values produce different answers for the question, and thus E should be the correct answer. The GMAC says A is the correct answer. Ok, here goes.

When x = 1 and y = 1, xy >0 (satisfies criteria in the question) and satisfies Statement I (1+1 = 1) and Statement II (1=1). Therefore (1-1)(1-1) does NOT equal 1.

Now, lets try different values.

When x = 2 and y =2, xy >0 (satisfies criteria in the question) and satisfies Statement I (2+2 = 4) and Statement II (2=2). Therefore (2-1)(2-1) DOES equal 1.

So, I think there is a problem here.

The brain has been hurting for a good 30 minutes now.

Thanks!

[Saurabh Malpani]

GMAT 5/18 I think it's just the day for both of us....let me show you your flaw!!! Pleas edon't do this on the exam day!!

but seriously it's just that it's 1 in the night!!! so...

One apply + one apple is ...????

Saurabh Malpani

....please stop the pain.

Source: GMAT Quant Review (Green Book) Official Guide, 11th Edition, Question 80, Data Sufficiency

I worked this problem, got the incorrect answer, reviewed the explanation, looked at my scribblings, and then came to the conclusion that the GMAC made a mistake. Then I thought, wait....the GMAC doesn't make mistakes, I do. So I repeated the steps above, and am still perplexed. So, if anyone can help, that would be awesome.

Here is the question and answers:

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

(1) x + y = xy
(2) x = y

Instead of writing my entire thoughts, I will just place 2 different values for x and y (that I feel satisfy both Statement I and II, as well as the question) - I believe these 2 values produce different answers for the question, and thus E should be the correct answer. The GMAC says A is the correct answer. Ok, here goes.

When x = 1 and y = 1, xy >0 (satisfies criteria in the question) and satisfies Statement I (1+1 = 1) and Statement II (1=1). Therefore (1-1)(1-1) does NOT equal 1.

Now, lets try different values.

When x = 2 and y =2, xy >0 (satisfies criteria in the question) and satisfies Statement I (2+2 = 4) and Statement II (2=2). Therefore (2-1)(2-1) DOES equal 1.

So, I think there is a problem here.

The brain has been hurting for a good 30 minutes now.

Thanks!

[GMAT 5/18]

It's a good thing I can laugh at myself!

Thanks, Saurabh!

[Saurabh Malpani]

hahaha I know it's just the day!!!

So how's ur prep till date!! you are native of..?

Saurabh Malpani

It's a good thing I can laugh at myself!

Thanks, Saurabh!

[GMAT 5/18]

Ummm, the prep is O.K. I guess. I think my biggest weakness in the quant section is knowing when to give up and not waste time. In all my practise tests I have taken I always run out of time and have to guess towards the end. So, my main focus of late is to begin eliminating early on the tough questions and make educated guesses throughout the exam rather than spend 3+ minutes on questions early and randomly guess towards the end. So yeah, prep is going good, just need to strategise better.

In the verbal, I think I am reasonably strong in the Reading Comprehension and Critical Reasoning, but comparatively weak in the Sentence Correction. Again, time is an issue for me, and I need to strategically guess.

I am a a native of (not from ) Australia. I am guessing you hail from India (just basing this off your name)?

[Saurabh Malpani]

Yes you guessed it right I am indeed from India ...so I am sure you my strengths and weaknesses.

I am doing decent in quant scoring regularly 49-51 in quant so. Thought timing is not an issue as such but yes on word problem it does becomes little tricky. So my apporach is that I ahve been following is what I call time monitor

when on question 10 my clock should not be less than 52 (23 mins for 10 questions). When on Question 20 not less than 30. Till question numbr 20 I make sure I don't make "errors" From this point onwards I take a call whether to guess or not. Say if a long convluted problem comes I will just blindly guess it and move forward so that I on track one DIFFICULT question is not going to hurt me.

What I follown on Verbal, which is my weak area, I make sure my strenghts SCs and CRs are correctly analyzed and if I find any Medecine related RC I will just randly select answers and move ( if it's not in first 20 questions) so basically I avoid wasting time on RC as my accuracy is not great on them

Are ou working in Kansas?

Ummm, the prep is O.K. I guess. I think my biggest weakness in the quant section is knowing when to give up and not waste time. In all my practise tests I have taken I always run out of time and have to guess towards the end. So, my main focus of late is to begin eliminating early on the tough questions and make educated guesses throughout the exam rather than spend 3+ minutes on questions early and randomly guess towards the end. So yeah, prep is going good, just need to strategise better.

In the verbal, I think I am reasonably strong in the Reading Comprehension and Critical Reasoning, but comparatively weak in the Sentence Correction. Again, time is an issue for me, and I need to strategically guess.

I am a a native of (not from ) Australia. I am guessing you hail from India (just basing this off your name)?

[Saurabh Malpani]

Sorry for the grammar ...I am just too sleepy....good night!

Take care
Saurabh Malpani

Yes you guessed it right I am indeed from India ...so I am sure you my strengths and weaknesses.

I am doing decent in quant scoring regularly 49-51 in quant so. Thought timing is not an issue as such but yes on word problem it does becomes little tricky. So my apporach is that I ahve been following is what I call time monitor

when on question 10 my clock should not be less than 52 (23 mins for 10 questions). When on Question 20 not less than 30. Till question numbr 20 I make sure I don't make "errors" From this point onwards I take a call whether to guess or not. Say if a long convluted problem comes I will just blindly guess it and move forward so that I on track one DIFFICULT question is not going to hurt me.

What I follown on Verbal, which is my weak area, I make sure my strenghts SCs and CRs are correctly analyzed and if I find any Medecine related RC I will just randly select answers and move ( if it's not in first 20 questions) so basically I avoid wasting time on RC as my accuracy is not great on them

Are ou working in Kansas?

Ummm, the prep is O.K. I guess. I think my biggest weakness in the quant section is knowing when to give up and not waste time. In all my practise tests I have taken I always run out of time and have to guess towards the end. So, my main focus of late is to begin eliminating early on the tough questions and make educated guesses throughout the exam rather than spend 3+ minutes on questions early and randomly guess towards the end. So yeah, prep is going good, just need to strategise better.

In the verbal, I think I am reasonably strong in the Reading Comprehension and Critical Reasoning, but comparatively weak in the Sentence Correction. Again, time is an issue for me, and I need to strategically guess.

I am a a native of (not from ) Australia. I am guessing you hail from India (just basing this off your name)?

[GMAT 5/18]

Yes, I am working in Kansas.

Wow, scoring 49-51 consistently is very impressive for Quant! I like your approach to the timing in regards to the Quant questions. Currently, I am going with:

After 7 questions, the clock should be no less than 60; after 14 questions, no less than 45, etc.

But there is too a fair bit of monitoring/calculating with my method, so I might copy yours (if you don't mind). 10 question time checks are easier I think. Also, I like the thinking on not making mistakes in the first 20 questions - I know MGMAT states that the first 10 questions or so don't matter more than the rest - but I think spending a bit more time on the first few questions has more to do than the GMAC algorithm - for me, spending a little more time at the beginning settles the nerves and helps to minimise the careless mistakes.

In regards to my guessing, I think I am going to make an educated "guess" on challenging combinatoric/probability questions reasonably quickly (as this is my weakest quant area) and spend a little more time on word problems (as they take the longest for me).

Goodnight, and good luck with the studying/prep.

[Saurabh Malpani]

Thanks Gmat 5/18.

Let me know if I can help in anyway in ur prep!! I know only 5 more days to go ..so nothing much can be done except revising but ..

Take care
Saurabh Malpani

Yes, I am working in Kansas.

Wow, scoring 49-51 consistently is very impressive for Quant! I like your approach to the timing in regards to the Quant questions. Currently, I am going with:

After 7 questions, the clock should be no less than 60; after 14 questions, no less than 45, etc.

But there is too a fair bit of monitoring/calculating with my method, so I might copy yours (if you don't mind). 10 question time checks are easier I think. Also, I like the thinking on not making mistakes in the first 20 questions - I know MGMAT states that the first 10 questions or so don't matter more than the rest - but I think spending a bit more time on the first few questions has more to do than the GMAC algorithm - for me, spending a little more time at the beginning settles the nerves and helps to minimise the careless mistakes.

In regards to my guessing, I think I am going to make an educated "guess" on challenging combinatoric/probability questions reasonably quickly (as this is my weakest quant area) and spend a little more time on word problems (as they take the longest for me).

Goodnight, and good luck with the studying/prep.

[OutOfTime]

Found your mistake! In your explanation you have written (1+1=1)!!! Dude! I know I have done such silly mistakes too!

Another way to solve this problem is algebrically.
Expand (x-1)(y-1)=1, you should get xy-y-x+1=1.
Now using statement 1, if you substitute value for xy in the above equation, you will see that statement is indeed sufficient.
Whereas if you do the substitution from statement 2 in the above equation, you will end up with a quadriatic equation, and is not sufficient.
So, answer choice A is correct and once again GMAC is right!

[dbernst]

Good work, all! And yes, even in that strange place know as "GMAT Land," 1 + 1 = 2!